Admob rewarded ads is not rewarding after being build

When I watching rewarded on Unity, there is no problem but after building when I watching rewarded, I don't get any rewards. I tried so hard and I can't find. Can you help me guys? Thank you in advance.

public class AdReward : MonoBehaviour
{
    private RewardedAd rewardedAd;
    int rewardAmount;
    int coinn;
    private void Start()
    {
        coinn= PlayerPrefs.GetInt("NumberOfCoins");
        string adUnitId;
#if UNITY_ANDROID
        adUnitId = "ca-app-pub-3940256099942544/5224354917";
#elif UNITY_IPHONE
            adUnitId = "ca-app-pub-3940256099942544/1712485313";
#else
            adUnitId = "unexpected_platform";
#endif
        MobileAds.Initialize(initStatus => { });
        this.rewardedAd = new RewardedAd(adUnitId);
        rewardedAd.OnUserEarnedReward += HandleUserEarnedReward;
        // Create an empty ad request.
        AdRequest request = new AdRequest.Builder().Build();
        // Load the rewarded ad with the request.
        this.rewardedAd.LoadAd(request);
    }
    public void HandleUserEarnedReward(object sender, Reward args)
    {
        double amount = args.Amount;
        coinn += (int)amount*20;
        PlayerPrefs.SetInt("NumberOfCoins", coinn);


    }
 
    public void UserChoseToWatchAd()
    {
        if (this.rewardedAd.IsLoaded())
        {
            this.rewardedAd.Show();
        }
    }
}