It seems that you're trying to link your Facebook/Google/Apple account from in-game settings, that's why the game says that this FB/Google/Apple account is already in-use (linked to NN profile).
Go to the main menu – Garage – Profile – Edit/switch profile and tap a "NEW PLAYER" button. On the following screen, choose "Sign in with Facebook/Google/Apple", that should allow you to choose your FB/Google profile and log in with it.