
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