No TestFlight invite email gets sent for internal users

Question

I set up my application for TestFlight. For external users it is waiting for review. But if I want to invite internal users, I select them and press invite. iTunes Connects says

Answer 1

This seems obvious, but I didn't see a main reply with this solution. Make sure your app is in "Beta" status (already through Beta review by Apple). If it is still in Prerelease status but has no beta, external users will never get in invitation email.