Make sure the following settings are enabled under Developer options with a Sim inserted:
Follow these steps:
Settings -> Additional Settings -> Developer options ->
Turn off "MIUI optimization" and Restart
Turn On "USB Debugging"
Turn On "Install via USB"
Set USB Configuration to Charging
Turn On "install via USB
and then check
Note: You must have the screen on the Home screen to see the install via USB popup. If you miss the popup then the app will not be installed.
If still not working then try the following options:
Sign in to MI Account
Go to Settings on the device
Tab on Additional Settings
Then Developer options
Near the bottom disable “Turn on MIUI Optimization”
If still not working → Sign out from MI Account and create a new account and enable USB Debugging