https://support.google.com/accounts/answer/9764949?p=ap-and&visit_id=637201535597737202-11306229&rd=1

To help protect against malware, new app installations from most sources outside the Play Store are blocked on any Android device that has a Google Account enrolled in Advanced Protection.

You’ll still be able to install Android apps from:

Google Play App stores that were pre-installed by your device manufacturer Android Debug Bridge (adb) Important: To install a new app from a different source, you’ll need to unenroll from Advanced Protection or remove the Advanced Protection-enrolled account from your device.