You cannot define a new game because your null level does not allow you to play more than 5 games at once and you are already playing (including your game invitations) 5 games.
You can solve this unpleasant situation by upgrading your membership.
(hide) If you don't want other users to know what you are doing you can change into cloak mode in Settings (paying members only). (pauloaguia) (show all tips)