Is the Bluetooth mbot chip paired with your Android?
See Makeblock Bluetooth Dongle Operation Guide | Makeblock Education . It discuses pairing. or use MakeBlocks instead of App Inventor. 
What Android version are you using. This discussion suggests the .the 2.0.4.aix extension on the mbot site does not work with Android 11+ .
This discussion might be useful Sending data to mBot via Bluetooth - mBot - Makeblock Forum See where one user used this MIT App Inventor Code on the Android:
BTArduinoClient2Devices_ahu.zip (3.1 KB) and he says You will need to pair the mBot with the Android device using the Bluetooth settings 1st, then connect within the app.