game crashes after adding adMob

Hi * I imported the AdMobUnityPackage today and then build and run it on my phone and the game crashes instantly. (doesn't even shows the "unity" screen). * I can't manage to fix it. please note that the game works fine in the editor, but not on the phone after I build it, in addition I have google play games in the project too, I heard that having Google Play Games may have influence if works along side adMob. really need your help! thanks!!