Cordova CLI v8.0 will use cordova-android@7.0.0 by default.
As most of the cordova plugins do not support cordova-android@7.0.0 yet, you can also try using the older one by specifying version:
cordova platform add android@6.4.0
Or, if you feel comfortable to use cordova-android@7.0.0, you can also wait for a update on cordova-plugi-admobpro to support the cordova-android@7.0.0, it’s coming soon.