my admob banner not showing,

i just want to request banner, this my script

     private BannerView bannerView;
 
     private AdRequest CreateAdRequest()
     {
         return new AdRequest.Builder()
             .Build();
     }
 
     public void RequestBanner()
     {
         // These ad units are configured to always serve test ads.
 #if UNITY_EDITOR
         string adUnitId = "unused";
 #elif UNITY_ANDROID
         string adUnitId = "ca-app-pub-3879637095276799/8119636209";
 #endif
 
         // Clean up banner ad before creating a new one.
         if (this.bannerView != null)
         {
             this.bannerView.Destroy();
         }
 
         // Create a 320x50 banner at the top of the screen.
         this.bannerView = new BannerView(adUnitId, AdSize.SmartBanner, AdPosition.Top);
 
         // Register for ad events.
         this.bannerView.OnAdLoaded += this.HandleAdLoaded;
         this.bannerView.OnAdFailedToLoad += this.HandleAdFailedToLoad;
         this.bannerView.OnAdOpening += this.HandleAdOpened;
         this.bannerView.OnAdClosed += this.HandleAdClosed;
         this.bannerView.OnAdLeavingApplication += this.HandleAdLeftApplication;
 
         // Load a banner ad.
         this.bannerView.LoadAd(this.CreateAdRequest());
         bannerView.Show();
     }
 
     #region Banner callback handlers
 
     public void HandleAdLoaded(object sender, EventArgs args)
     {
         MonoBehaviour.print("HandleAdLoaded event received");
     }
 
     public void HandleAdFailedToLoad(object sender, AdFailedToLoadEventArgs args)
     {
         MonoBehaviour.print("HandleFailedToReceiveAd event received with message: " + args.Message);
     }
 
     public void HandleAdOpened(object sender, EventArgs args)
     {
         MonoBehaviour.print("HandleAdOpened event received");
     }
 
     public void HandleAdClosed(object sender, EventArgs args)
     {
         MonoBehaviour.print("HandleAdClosed event received");
     }
 
     public void HandleAdLeftApplication(object sender, EventArgs args)
     {
         MonoBehaviour.print("HandleAdLeftApplication event received");
     }
 
     #endregion

i have download plugin and anything, but still not showing,, maybe can you give me a simple project in which there is a plugin, simple script to request banner, I've tried to solve this problem in two days, but still can't work, pleae help,,,,