Using Classic Outlook on Windows for personal email, calendar, and contact management
Just exit full screen mode, that will allow you to access the pop-up window.
I have the same problem, and I am not using the full screen mode.
I had this issue with the spell checker in the new free version of Outlook a few months ago. So, I went back to using Microsoft Mail. Then I have tried the new Outlook again and this issue appeared to have been solved. I have managed to add additional language and it has stopped underlining in red every word I have typed in languages other than English. However, this problem has returned a few days ago. This is really difficult to understand... Now there is no way of going back even to Microsoft Mail, as I think it has been discontinued.