Has the npm worked before?
I always use the git repo.
Plugin management > 3rd party > git repo > add the address to the 1st box > click on the add variable (i think its called) on the bottom. 2 boxes will appear. In the far right box add your admob app id.
Sorry, its late here. Tomorrow i will post the git repo address i use. This always works for me.
Also, when i have to build a newer test version (same project or not) i ALWAYS delete the 3rd party plugins i added before. I've had errors before if i leave them, so to make sure they work correctly, i delete then add them again.