Unity 5.5 ads not working

Hello Friends, I have integrated Unity ad in my android game but my ad is not showing in the editor or in the device itself.I have attached the script to the main camera of my scene.

This is the Script is -

using System.Collections; using System.Collections.Generic; using UnityEngine; using UnityEngine.Advertisements;

public class AdScript : MonoBehaviour {

 public void Start(){
     if (Advertisement.IsReady ()) {
         Advertisement.Show ();
     }
 }

}