How to load an InterstitialAd properly?

Error message:

-Assets\Replay.cs(22,33): error CS1729: 'InterstitialAd' does not contain a constructor that takes 1 arguments -Assets\Replay.cs(26,27): error CS1061: 'InterstitialAd' does not contain a definition for 'LoadAd' and no accessible extension method 'LoadAd' accepting a first argument of type 'InterstitialAd' could be found (are you missing a using directive or an assembly reference?) Assets\Replay.cs(31,31): error CS1061: 'InterstitialAd' does not contain a definition for 'IsLoaded' and no accessible extension method 'IsLoaded' accepting a first argument of type 'InterstitialAd' could be found (are you missing a using directive or an assembly reference?)

Replay.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.SceneManagement;
using GoogleMobileAds.Api;

public class Replay : MonoBehaviour
{
    private InterstitialAd interstitial;

    private void RequestInterstitial()    
    {
        #if UNITY_ANDROID
            string adUnitId = "ca-app-pub-3940256099942544/1033173712";
        #elif UNITY_IPHONE
            string adUnitId = "ca-app-pub-3940256099942544/4411468910";
        #else
            string adUnitId = "unexpected_platform";   
        #endif

        // Initialize an InterstitialAd.
        this.interstitial = new InterstitialAd(adUnitId);
        // Creates an empty ad request.
        AdRequest request = new AdRequest.Builder().Build();
        // Load the interstitial with the request
        this.interstitial.LoadAd(request);     
    }
    public void ReplayGame()
    {
        RequestInterstitial();
        if (this.interstitial.IsLoaded()) {
            this.interstitial.Show();
        }   
        //SceneManager.LoadScene("PlayScene");
    }
}

I don’t know which part to correct… Help me~~