User cannot download App


I have shared my App with a test user by sending them an email invitation through Appsheet.

When they click on the install button they say they see a grey screen with a lock icon on it.

I don’t understand why this would be. I have tested installing the App myself and it works fine. We are both using iOS and chrome browser. I have enabled the security option “Allow all signed-in users”. My App is in a Not Deployed state, but my understanding is this should not matter as up 10 users can trial the App while in this state.

Any help would be much appreciated!