admob banner doesn't show up.

Hi.I did everything correctly and the ad isn't show up at all. i imported the admob plugin , i created a script and i atached it to an empty game object. I ve used my admob banner string,but still nothing shows up i ve tried to see if it is invisible but it's not.

Here is the code: using System.Collections; using System.Collections.Generic; using UnityEngine; using GoogleMobileAds.Api; public class Ads : MonoBehaviour {

 // Use this for initialization
 void Start () {

     showBannerAd();
 }

 private void showBannerAd()
 {
     string adID = " ca-app-pub-1366562434495504/4299282974";

     AdRequest request = new AdRequest.Builder().Build();

     BannerView bannerAd = new BannerView(adID, AdSize.SmartBanner, AdPosition.Top);
     bannerAd.LoadAd(request);
 }
 // Update is called once per frame
 void Update () {
     
 }

}