Advertisement.IsReady(). my button's interactable is always false

I just want to make this; if an ad is ready to show than user should be available to click this button but this code doesn't make that happen. Please share your knowledge

Code (CSharp):

1.  public Text points;
2.     public static int points1;
3.     public Button watchbutton;
4.      void Start()
5.     {
6.        
7.         points.text="Points: "+PlayerPrefs.GetInt("ScoreCmon").ToString();
8.         if (UnityEngine.Advertisements.Advertisement.IsReady("rewardedVideo") == false)
9.         {
10.  
11.             watchbutton.interactable = false;
12.             watchbutton.image.color = Color.red;
13.         }
14.  
15.  
16.     }
17.      void Update()
18.     {
19.         points.text = "Points: " + PlayerPrefs.GetInt("ScoreCmon").ToString();
20.      
21.     }
22.     public void watch()
23.     {
24.  
25.        
26.             UnityEngine.Advertisements.Advertisement.Show("rewardedVideo", new ShowOptions() { resultCallback = HandleAdResult });
27.        
28.  
29.  
30.        
31.     }
32.     private void HandleAdResult(ShowResult result)
33.     {
34.  
35.         switch (result)
36.         {
37.             case ShowResult.Finished:
38.                points1= PlayerPrefs.GetInt("ScoreCmon") + 100;
39.                 PlayerPrefs.SetInt("ScoreCmon",points1);
40.                 Time.timeScale = 1f;
41.  
42.                 break;
43.             case ShowResult.Skipped:
44.                
45.                 break;
46.             case ShowResult.Failed:
47.                
48.                 break;
49.         }
50.     }