If you can still play the game, sounds like you're already registered for EW. PS4/5 user register automatically, Windows via Square Store or Steam request second official code to be registered, Mac I believe is same as Windows. You trying to reregister something you already have registered can cause an error. If you SE account at the Mog Station shows Endwalker in the list of register expansion and you're still able to play sounds like you're registered. That just my guess.