I have problem admob reference misssing

I install google admob sdk from this url Release Google Mobile Ads Unity Plugin v5.1.0 · googleads/googleads-mobile-unity · GitHub.

But the using GoogleMobileAds.Api; reference is missing.

using System;
using UnityEngine;
using GoogleMobileAds.Api; //THIS REFERENCE MISSING

public class AdmobManager : MonoBehaviour
{
    private InterstitialAd interstitial;
    private BannerView bannerView;
    public void Start()
    {
#if UNITY_ANDROID
        string appId = "ca-app-pub-5911743213156480~5159541036";
#elif UNITY_IPHONE
            string appId = "ca-app-pub-3940256099942544~1458002511";
#else
            string appId = "unexpected_platform";
#endif

        // Initialize the Google Mobile Ads SDK.
        MobileAds.Initialize(appId);

        RequestInterstitialAds();
        RequestBanner();
    }

    private void RequestBanner()
    {


#if UNITY_ANDROID
        string adUnitId = "ca-app-pub-5911743213156480/9909590242";
#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.
        bannerView = new BannerView(adUnitId, AdSize.SmartBanner, AdPosition.Bottom);
        // Create an empty ad request.


        // Load the banner with the request.
        bannerView.LoadAd(CreateNewRequest());
    }
    public void ShowInterstitialAds()
    {
        if (interstitial.IsLoaded())
        {
            interstitial.Show();
        }
    }
    public void ShowBannerAd()
    {
        if (bannerView != null)
        {
            bannerView.Show();
        }
    }
    private void RequestInterstitialAds()
    {
#if UNITY_ANDROID
        string adUnitId = "ca-app-pub-5911743213156480/3275382212";
#elif UNITY_IPHONE
                    string adUnitId = "ca-app-pub-3940256099942544/2934735716";
#else
                    string adUnitId = "unexpected_platform";
#endif
        interstitial = new InterstitialAd(adUnitId);
        interstitial.LoadAd(CreateNewRequest());
    }


    private AdRequest CreateNewRequest()
    {
        return new AdRequest.Builder().Build();
    }
}

Someone can help me?

Hi, had this with latest unity admob.

remove current installation resolved & installed files, and then install the previous release.

For me the one on 18 - Mar is working.