Hi John,
This doesn't look like an AdMob error, but I did some research and it looks like it pops up from time to time when people try to compile GameMaker with our SDK (I also saw your post on the YoYo Games forum). From other posts online, it looks like people have been able to get past the error by making sure they're using the latest version of Build Tools.
In your manifest file try adding xmlns:tools="http://schemas.android.com/apk/res-auto" in your manifest tag, then add in your application tag tools:replace="label".
Let me know if you continue to have issues.
Regards,
Chris Feldman
Mobile Ads SDK Team