UnityAds work in Editor, but not on the device

using UnityEngine;

using UnityEngine.SceneManagement;

using UnityEngine.UI;

using UnityEngine.Advertisements;

public class BallScript : MonoBehaviour {

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

public Vector2 startForce;

 static int times = 0;

 public Text goverText;

 public Vector3[] positions;

 public Rigidbody2D rb;

 void Start()
 {
     int randomNumber = Random.Range(0, positions.Length);
     transform.position = positions[randomNumber];
     times = PlayerPrefs.GetInt("Played:", times);
     rb.AddForce(startForce, ForceMode2D.Impulse);


 }

 void Update()
 {

     PlayerPrefs.SetInt("Played:", times);
     goverText.text = "Played:" + times.ToString();


 }

 void OnCollisionEnter2D(Collision2D other)
 {
     if (other.gameObject.name == "wall_bottom")
     {

         times++;

         if (times==19)
         {
             ShowAd();
         }
           
         SceneManager.LoadScene("wfvv");


     }

 }

}