I resolved. I’ve rewrote step by step the code for AdMob (It’s important because I don’t know why but if you simply copy and paste it doesn’t work) following the already made code from this link: How to make AdMob work with the latest MG?
VERY IMPORTANT: You have to add the following pakages via NuGet:
Xamarin Google Play Services Base
Xamarin Google Play Services Ads
Xamarin Support v4