I had the same issue. To make it work I had to use godot 3.2.2 and the ShinNil plugin release 4.0.0. Extract the contents of the admob plugin file (inside the zip) to res/android/plugin directory of your godot project. Extract the admob-lib folder to your projects res// folder. Follow the instructions mentioned in the github page about how to change the android manifest.xml file. DO NOT type anything in the module section of android sub-section of project settings in godot. Leave it empty as it is. Copy and paste the test ad id's to the admob node. and export to your android device. It worked for me. I don't know why it worked though.