UnityAds - Ads not showing

Hello!

Basically I have a problem with my UnityAds plugin. You see, the ads won't show. At all. I have read the documentation, and followed a youtube video - showing you how to set up and initialize the ads. Here is what my code looks like, right now;

 void Awake() {
         if (Advertisement.isSupported) {
             Advertisement.allowPrecache = true;
             Advertisement.Initialize ("MYUNIQUEID");
         } else {
             Debug.Log("Platform not supported");
         }
     }
 public void viewAd() {
 if(Advertisement.isReady()){
     Advertisement.Show(); 
    }
 }

I have also tried this code, without any luck;

 if(GUI.Button(new Rect(10, 10, 150, 50), Advertisement.isReady() ? "Show Ad" : "Waiting...")) {
             // Show with default zone, pause engine and print result to debug log
             Advertisement.Show(null, new ShowOptions {
                 pause = true,
                 resultCallback = result => {
                     Debug.Log(result.ToString());
                 }
             });

In my console, I just get a message saying "failed". Do I need to configure something else? I have seen in other threads that people are talking about a prefab - but I have searched, and I am almost certain that I didn't get any prefab installed. Help would me much appreciated, as I have tried to solve this for what feels like days now.

Thanks in advance.