As the error states, the OnAdLoaded method, when overridden provides a parameter of type Java.Lang.Object. Similarly, OnAdFailedToLoad provides a parameter of type LoadAdError

It seems you're trying to implement Interstitial Ads in your app and you are expecting the provided parameter of OnAdLoaded method to be of type InterstitialAd.

I would suggest to follow the approach specified in this link to achieve this.

You just have to create a custom callback class instead of directly using InterstitialAdLoadCallback to get InterstitialAd as parameter to OnAdLoaded method.