[Unity game admob] Why interstitial ad doesn't work?

I would like to put ads in my Unity game when OverSystem appears to me.

Unfortunately, interstitial ad doesn't work. Any help?

     private InterstitialAd interstitial;
 
     
     void Start ()
     {
         RequestInterstitial() ;
         GetComponent<AudioSource> ().playOnAwake = false;
         GetComponent<AudioSource> ().clip = destroySound;
 
     }
     public void OnCollisionEnter2D (Collision2D col)
     {
         
         collisionObject colObj= col.transform.GetComponent<collisionObject>();
 
         if(!colObj) return;
         CheckDestroyedIndex(colObj);
 
 
     }
     void CheckDestroyedIndex(collisionObject cObj) {
 
         if(index == cObj.myIndexNumber)
 
         {
             if (cObj.isHighestNumber)
 
             {
 
                 Debug.Log ("Yay you correctly hit the last Square, You WIN!");
                 WinSystem.SetActive (true);
                 gameObject.SetActive (false);
                 Manager.SetActive (false);
 
             }
 
             Destroy(cObj.gameObject);
             index++;
 
         }
         
         else
         {
 
             GameOver ();
             {
                 if (interstitial.IsLoaded()) {
                     interstitial.Show();
                 }
             }
         }
     }
 
     void GameOver (){
         ImageOver.SetActive (true);
         restartButton.SetActive (true);
         menuButton.SetActive (true);
         gameObject.SetActive (false);
         Manager.SetActive (false);
         WinSystem.SetActive (false);
     }
 
 
     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
 
 
         // Clean up interstitial ad before creating a new one.
         if (this.interstitial != null) {
             this.interstitial.Destroy ();
         }
         // Initialize an InterstitialAd.
         InterstitialAd interstitial = new InterstitialAd(adUnitId);
         // Create an empty ad request.
         AdRequest request = new AdRequest.Builder().Build();
         // Load the interstitial with the request.
         interstitial.LoadAd(request);
 
 
         // Called when an ad request has successfully loaded.
         interstitial.OnAdLoaded += HandleOnAdLoaded;
         // Called when an ad request failed to load.
         interstitial.OnAdFailedToLoad += HandleOnAdFailedToLoad;
         // Called when an ad is shown.
         interstitial.OnAdOpening += HandleOnAdOpened;
         // Called when the ad is closed.
         interstitial.OnAdClosed += HandleOnAdClosed;
 
         }
 
         public void HandleOnAdLoaded(object sender, EventArgs args)
         {
         MonoBehaviour.print("HandleAdLoaded event received");
         }
 
         public void HandleOnAdFailedToLoad(object sender, AdFailedToLoadEventArgs args)
         {
         MonoBehaviour.print("HandleFailedToReceiveAd event received with message: "
         + args.Message);
         }
 
         public void HandleOnAdOpened(object sender, EventArgs args)
         {
         MonoBehaviour.print("HandleAdOpened event received");
         }
 
         public void HandleOnAdClosed(object sender, EventArgs args)
         {
         MonoBehaviour.print("HandleAdClosed event received");
         }
         }