I'm having this same issue trying to create the Super Smash Bros. Ultimate leaderboard, as one of the other mods done this a few weeks ago without realizing the 2 week prior to release policy, it now shows us the invalid game error when we try to "recreate" it in a sense.

I believe after the server rollback series mods are no longer able to make new boards atm

I got the same issue today for adding a game for another user. I will wait for the staff fix that.

It's a "bug" due to server backup or staff have temporarily disabled the game addition for series mods due to Nov. 16 issues ?

I sent a message to the full mods and got the issue fixed a couple hours ago, enjoy adding games to series again!