Anyone else who has reached this point and is still pulling out their hair.... in App Store Connect, it's not enough to just have set up the leaderboards on the 'Features' tab. You also need to link the leaderboard to the app on the 'App Store' tab too. Tick the Game Center tick box halfway down the page and then select the leaderboards to include in this app version.

Unfortunately the whole Game Centre pages in App Store Connect are pretty terrible :-/. Well due for some attention Apple. I'm on a 2020 M1 MacBook Air using Safari in full screen mode and I'm unable to view the popup when I try to add another language. One tip I can provide that does work is to toggle the maximise window button on the browser (e.g. Safari). I find this is often enough to redraw the screen and bring the popup to actually being displayed on-screen. Good luck!