showListener is null, you will not receive any callbacks UnityEngine.Advertisements.Advertisement:Show(String)

using System;
using UnityEngine;
using GoogleMobileAds.Api;

using UnityEngine.Advertisements;
using UnityEngine.SceneManagement;

public class Admob_Unity_Ads : MonoBehaviour , IUnityAdsInitializationListener , IUnityAdsLoadListener, IUnityAdsShowListener
{
[Header(“Test Mode”)]
public bool Test = true;

private RewardedAd rewarded_ad;
public BannerView bannerView;

public string Andriod_Banner_ID;
public string rewarded_ad_id;
public string Interstitial_ad_id;
public string Rew_Interstitial_ad_id;

private InterstitialAd interstitial;

private bool Req_Interstitial_Add = true;

private RewardedInterstitialAd rewardedInterstitialAd;

[Header("Unity Test Mode")]
public bool TestMode;
// For Unity
[Header("Unity ID's")]
public string gameID = "";
public string bannerID = "Banner_Android";
public string interstitialID = "Interstitial_Android";
public string rewardedVideoID = "Rewarded_Android";

void Start()
{

    //rewarded_ad_id = "ca-app-pub-5616490787498850/9447604557";

    MobileAds.Initialize(InitializationStatus => { });

    //  requestintertetial();
    requestrewardedvideo();
    Show_Ban();
    Rew_Int_Request();
    if (Req_Interstitial_Add == true)
    {
        this.RequestInterstitial();
        Req_Interstitial_Add = false;
    }

    //For Unity
    //Advertisement.Initialize(gameID, TestMode);
    InitializeAds();
    //this.Advertisement.AddListener;

}
public void InitializeAds()
{
    Advertisement.Initialize(gameID, TestMode, this);
}
public void Rew_Int_Request()
{
    // Create an empty ad request.
    AdRequest request = new AdRequest.Builder().Build();
    // Load the rewarded ad with the request.
    RewardedInterstitialAd.LoadAd(Rew_Interstitial_ad_id, request, adLoadCallback);
}

public void ShowRewardedInterstitialAd()
{

    if (rewardedInterstitialAd != null)
    {
        rewardedInterstitialAd.Show(userEarnedRewardCallback);
    }
    else
    {
        Advertisement.Show(interstitialID);
    }
  

}

private void adLoadCallback(RewardedInterstitialAd ad, string error)
{
    if (error == null)
    {
        rewardedInterstitialAd = ad;

        rewardedInterstitialAd.OnAdFailedToPresentFullScreenContent += HandleAdFailedToPresent;
        rewardedInterstitialAd.OnAdDidPresentFullScreenContent += HandleAdDidPresent;
        rewardedInterstitialAd.OnAdDidDismissFullScreenContent += HandleAdDidDismiss;
        rewardedInterstitialAd.OnPaidEvent += HandlePaidEvent;
    }
}
private void HandleAdFailedToPresent(object sender, AdErrorEventArgs args)
{
     print("Rewarded interstitial ad has failed to present.");
}

private void HandleAdDidPresent(object sender, EventArgs args)
{
    bannerView.Hide();
    print("Rewarded interstitial ad has presented.");
}

private void HandleAdDidDismiss(object sender, EventArgs args)
{
    print("Rewarded interstitial ad has dismissed presentation.");
    bannerView.Show();

}

private void HandlePaidEvent(object sender, AdValueEventArgs args)
{
    MonoBehaviour.print(
        "Rewarded interstitial ad has received a paid event.");
}

private void userEarnedRewardCallback(Reward reward)
{
    // TODO: Reward the user.
}

public void RequestInterstitial()
{
    string adUnitId = "";

    if (Test)
    {
        if (Application.platform == RuntimePlatform.Android)
        {
            adUnitId = "ca-app-pub-3940256099942544/1033173712";
        }
    }
    else
    {
        if (Application.platform == RuntimePlatform.Android)
        {
            adUnitId = Interstitial_ad_id;
        }
        //if (Application.platform == RuntimePlatform.IPhonePlayer)
        //{
        //    //adUnitId = IOS_Intertisial_ID;
        //}
    }

    //#if UNITY_ANDROID
    //        string adUnitId = Interstitial_ad_id;
    //#elif UNITY_IPHONE
    //        string adUnitId = "ca-app-pub-3940256099942544/4411468910";
    //#else
    //        string adUnitId = "unexpected_platform";
    //#endif

    // Initialize an InterstitialAd.
    this.interstitial = new InterstitialAd(adUnitId);

    // Called when an ad request has successfully loaded.
    this.interstitial.OnAdLoaded += HandleOnAdLoaded;
    // Called when an ad request failed to load.
    this.interstitial.OnAdFailedToLoad += HandleOnAdFailedToLoad;
    // Called when an ad is shown.
    this.interstitial.OnAdOpening += HandleOnAdOpened;
    // Called when the ad is closed.
    this.interstitial.OnAdClosed += HandleOnAdClosed;
    // Called when the ad click caused the user to leave the application.
    this.interstitial.OnAdLeavingApplication += HandleOnAdLeavingApplication;

    // Create an empty ad request.
    AdRequest request = new AdRequest.Builder().Build();

    // Load the interstitial with the request.
    this.interstitial.LoadAd(request);
}

public void RequestBanner()
{
    string adUnitId = "";
    if (Test)
    {
        if (Application.platform == RuntimePlatform.Android)
        {
            adUnitId = "ca-app-pub-3940256099942544/6300978111";
        }
        if (Application.platform == RuntimePlatform.IPhonePlayer)
        {
            //adUnitId = "ca-app-pub-3940256099942544/2934735716";
        }
    }
    else
    {
        if (Application.platform == RuntimePlatform.Android)
        {
            adUnitId = Andriod_Banner_ID;
        }
        if (Application.platform == RuntimePlatform.IPhonePlayer)
        {
            //adUnitId = IOS_Banner_ID;
        }
    }

    //#if UNITY_ANDROID
    //        //string adUnitId = "ca-app-pub-5616490787498850/7462964263";  
    //        //adUnitId = Andriod_Banner_ID;
    //#elif UNITY_IPHONE
    //            string adUnitId = "ca-app-pub-3940256099942544/2934735716";
    //#else
    //        string adUnitId = "unexpected_platform";
    //#endif

    //Create a 320x50 banner at the top of the screen.
    this.bannerView = new BannerView(adUnitId, AdSize.SmartBanner, AdPosition.Top);

    //AdSize adSize = new AdSize(250, 250);
    // this.bannerView = new BannerView(adUnitId, adSize, AdPosition.Top);

    //Create a 320x50 banner ad at coordinate(0, 50) on screen.
     //AdSize adSize = new AdSize(11, 20);
    //this.bannerView = new BannerView(adUnitId, adSize, AdPosition.Top);

    // Create an empty ad request.
    AdRequest request = new AdRequest.Builder().Build();

    // Load the banner with the request.
    this.bannerView.LoadAd(request);

    // Called when an ad request has successfully loaded.
    this.bannerView.OnAdLoaded += this.HandleOnAdLoaded;
     // Called when an ad request failed to load.
    this.bannerView.OnAdFailedToLoad += this.HandleOnAdFailedToLoad;
     // Called when an ad is clicked.
    this.bannerView.OnAdOpening += this.HandleOnAdOpened;
     // Called when the user returned from the app after an ad click.
    this.bannerView.OnAdClosed += this.HandleOnAdClosed;
     // Called when the ad click caused the user to leave the application.
    this.bannerView.OnAdLeavingApplication += this.HandleOnAdLeavingApplication;
 }

private void HandleOnAdLoaded(object sender, EventArgs args)
{

}

private void HandleOnAdFailedToLoad(object sender, AdFailedToLoadEventArgs args)
{

}

private void HandleOnAdOpened(object sender, EventArgs args)
{
    bannerView.Destroy();
}

private void HandleOnAdClosed(object sender, EventArgs args)
{
    Show_Ban();
 
}
private void HandleOnAdClosed1(object sender, EventArgs args)
{
   
 
}

private void HandleRewardedAdClosed(object sender, EventArgs args)
{
    requestrewardedvideo();
    Show_Ban();
}

private void HandleOnAdLeavingApplication(object sender, EventArgs args)
{

}
//public void HandleOnUserEarnedReward(object sender, EventArgs args)
//{
//    requestrewardedvideo();
//    string type = args.Type;
//    double amount = args.Amount;
//    MonoBehaviour.print(
//        "HandleRewardedAdRewarded event received for "
//                    + amount.ToString() + " " + type);

//}

public void HandleOnUserEarnedReward(object sender, Reward args)
{
    requestrewardedvideo();
    string type = args.Type;
    double amount = args.Amount;
    MonoBehaviour.print(
        "HandleRewardedAdRewarded event received for "
                    + amount.ToString() + " " + type);
    PlayerPrefs.SetInt("coins", PlayerPrefs.GetInt("coins") + 200);
}
public void HandleRewardedAdFailedToShow(object sender, AdErrorEventArgs args)
{
    requestrewardedvideo();

}

void Update()
{
    if (Req_Interstitial_Add == true)
    {
        this.RequestInterstitial();
        Req_Interstitial_Add = false;
    }
}

private void requestrewardedvideo()
{
    string adUnitId = "";

    if (Test)
    {
        if (Application.platform == RuntimePlatform.Android)
        {
            adUnitId = "ca-app-pub-3940256099942544/5224354917";
        }
        //if (Application.platform == RuntimePlatform.IPhonePlayer)
        //{
        //    adUnitId = "ca-app-pub-3940256099942544/1712485313";
        //}
    }
    else
    {
        if (Application.platform == RuntimePlatform.Android)
        {
            adUnitId = rewarded_ad_id;
        }
        //if (Application.platform == RuntimePlatform.IPhonePlayer)
        //{
        //    adUnitId = IOS_Rewarded_ID;
        //}
    }

    rewarded_ad = new RewardedAd(rewarded_ad_id);
    rewarded_ad.OnUserEarnedReward += HandleOnUserEarnedReward;
    rewarded_ad.OnAdClosed += HandleRewardedAdClosed;
    rewarded_ad.OnAdFailedToShow += HandleRewardedAdFailedToShow;
    AdRequest request = new AdRequest.Builder().Build();
    rewarded_ad.LoadAd(request);

}

public void showrewardedvideo()
{
    if (this.rewarded_ad.IsLoaded())
    {
        Destroy_Banner();
        this.rewarded_ad.Show();
    }
    else
    {
        Advertisement.Show(rewarded_ad_id);
    }
  

}
public void Destroy_Banner()
{
    bannerView.Destroy();

}

private void Show_Ban()
{
    this.RequestBanner();

}
public void Show_Interstitial_Ad()
{

    //if (this.interstitial.IsLoaded())
    //{

    //    this.interstitial.Show();
    //    Req_Interstitial_Add = true;

    //}
    //else
    {
        Advertisement.Show(interstitialID);
    }

}

//For Unity

public void ShowInterstitial()
{
    
        Advertisement.Show(interstitialID);
   
}

public void ShowRewardedVideo()
{
    Advertisement.Show(rewardedVideoID);
}

public void HideBanner()
{
    Advertisement.Banner.Hide();
}

public void OnUnityAdsReady(string placementID)
{
    if (placementID == rewardedVideoID)
    {
        //watchRewardAd.interactable = true;
    }

    if (placementID == interstitialID)
    {
        //showInterstitial.interactable = true;
    }

    //if (placementID == bannerID)
    //{
    //    Advertisement.Banner.SetPosition(BannerPosition.TOP_CENTER);
    //    Advertisement.Banner.Show(bannerID);
    //}
}

public void OnUnityAdsDidFinish(string placementID, ShowResult showResult)
{
    if (placementID == rewardedVideoID)
    {
        if (showResult == ShowResult.Finished)
        {
            GetReward();
        }
        else if (showResult == ShowResult.Skipped)
        {
            //Do nothing
        }
        else if (showResult == ShowResult.Failed)
        {
            //tell player ads failed
        }
    }
}

public void OnUnityAdsDidError(string message)
{
    //Show or log the error here
}

public void OnUnityAdsDidStart(string placementID)
{
    //Do this if ads starts
}

private void GetReward()
{
    if (PlayerPrefs.HasKey("coins"))
    {
        int gemAmount = PlayerPrefs.GetInt("coins");
        PlayerPrefs.SetInt("coins", gemAmount + 200);
    }
    else
    {
        PlayerPrefs.SetInt("coins", 200);
    }

    //gemsAmt.text = "coins: " + PlayerPrefs.GetInt("coins").ToString();
}

public void OnInitializationComplete()
{
   // DebugLog("Init Success");
}

public void OnInitializationFailed(UnityAdsInitializationError error, string message)
{
  //  DebugLog($"Init Failed: [{error}]: {message}");
}

public void OnUnityAdsAdLoaded(string placementId)
{
   // DebugLog($"Load Success: {placementId}");
}

public void OnUnityAdsFailedToLoad(string placementId, UnityAdsLoadError error, string message)
{
    //DebugLog($"Load Failed: [{error}:{placementId}] {message}");
}

public void OnUnityAdsShowFailure(string placementId, UnityAdsShowError error, string message)
{
    //DebugLog($"OnUnityAdsShowFailure: [{error}]: {message}");
}

public void OnUnityAdsShowStart(string placementId)
{
   // DebugLog($"OnUnityAdsShowStart: {placementId}");
}

public void OnUnityAdsShowClick(string placementId)
{
   // DebugLog($"OnUnityAdsShowClick: {placementId}");
}

public void OnUnityAdsShowComplete(string placementId, UnityAdsShowCompletionState showCompletionState)
{
   // DebugLog($"OnUnityAdsShowComplete: [{showCompletionState}]: {placementId}");
}

//#endregion
}

This issue is due to you have not added any Interface listener for Show method of Advertisement.

Resolve: add this for Show method of Advertisement like as below:
Advertisement.Show(rewardedVideoID, this);
Advertisement.Show(interstitialID, this);