(Ads) initializationListener is null, you will not receive any callbacks

The title explains it all, any idea why this could be happening & how I can fix it?
Full error:

initializationListener is null, you
will not receive any callbacks
UnityEngine.Advertisements.Advertisement:Initialize
(string)

@BranditTheBandit
With Unity Ads, if you want it to tell you when it has finished or failed initialization. you need to give it a class instance that can receive the messages.

This class needs to implement the initializationListener interface.

The interface can be implemented in the same class you are calling the initialize routine from, or another class - something like this.

     public class MyClass : IUnityAdsInitializationListener
        {

            // Called from outside this class
            public void Initialize()
            {
                   Advertisement.Initialize(advertisingID, testMode, this);
            }

            void OnInitializationComplete()
             {
                   // Do something
             }
             
             void OnInitializationFailed(UnityAdsInitializationError error, string message);
             {
                  // Do Something
             }
        
        }

Its all in the SDK reference guide: Unity Ads

All the call backs in Unity ads (ad loaded, shown, etc) are achieved via various interfaces. The same class can implement all of them if you wish.

Zzodo Games