I have problem admob reference misssing

I install google admob sdk from this url https://github.com/googleads/googleads-mobile-unity/releases/tag/v5.1.0.

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?