Plugin ‘firebase_app_distribution’ was not properly loaded

Looks like some kind of file permission issue under Catalina, so chmod might help. But you can also install fastlane-plugin-firebase_app_distribution in gem’s USER INSTALLATION DIRECTORY (gem env will tell you where it is).

Uninstall the gem from the default dir:

sudo gem uninstall fastlane-plugin-firebase_app_distribution

Install in user dir:

gem install fastlane-plugin-firebase_app_distribution --user-install

https://stackoverflow.com/a/61914727