iOS9 “This game is not recognized by game center.”

Adding a leaderboard fixed the problem even though I wasn't using the leaderboard.

Did you enable Game Center in the project's capabilities?

Have you added the app in iTunesConnect?

Have you uploaded a build to iTunesConnect? If so, a good check to validate the that your setup is correct is to go to Activity | All Builds | ~your current build~, then scroll down to Required Capabilities. Does it list "gamekit" in that section?

The latest answer to the game is not recognized by game center suggests that if everything else is correct, just adding an achievement (or maybe a leaderboard) cleared the issue.

Edit: since writing this, the problem has appeared a few times for me. Creating a dummy leaderboard solved it the first time. Creating yet another solved it the second time. The third time it happened, just changing which of the dummy leaderboards was the default solved the problem.

Edit 2: Based on How to list all available GKTurnBasedMatches for a player?, it appears that in some cases, at least using Swift, you don't even get the "unrecognized" error. Operations just silently fail until you create a leaderboard.

There is a toggle switch in appstoreconnect.

In your "Prepare for submission" tab, toggle this switch:

I solved this by creating a new leaderboard in iTunes Connect