Unity Ads using monetization sdk 3.0 not working

I downloaded the unity monetization 3.0 and the ads work in the inspector but I get these errors: Initializing Unity Ads. unityengine.Advertisements.Creator.LoadRuntime() and ArgumentNullException: Value cannot be null. parameter name: type.

The ads work in the inspector but not when I build to IOS or android, here is my script:

 public string placementId = "rewardedVideo";

 private bool testMode = false;

 private string gameId = "1234567";

 private GameManager gameManager;

 void Start()
 {
     gameManager = FindObjectOfType<GameManager>();

     if(Application.platform== RuntimePlatform.IPhonePlayer)
     {
         gameId = "1234567";
     }

     else if (Application.platform == RuntimePlatform.Android)
     {
         gameId = "7654321";
     }

     if (Monetization.isSupported)
     {
         Monetization.Initialize(gameId, testMode);
     }       
 }

 public void ShowAd()
 {
     StartCoroutine(WaitForAd());
 }

 IEnumerator WaitForAd()
 {
     while (!Monetization.IsReady(placementId))
     {
         yield return null;
     }

     ShowAdPlacementContent ad = null;
     ad = Monetization.GetPlacementContent(placementId) as ShowAdPlacementContent;

     if (ad != null)
     {
         ad.Show(AdFinished);
     }
 }

 void AdFinished(ShowResult result)
 {
     if (result == ShowResult.Finished)
     {
         GameData.gameData.saveData.lives++;

         if (gameManager != null)
         {
             gameManager.LoadData();
         }
     }
 }

I have ads turned on and I put my games ID in the game ID string. I am using the latest version of unity. when I click on the ad button on android device after its built nothing happens and the add does not launch. is there a reason it would work in the inspector and not when I build the game? Thanks