If you try to log in to the game via the guest account while you don't enter the game that is logged in with the linked account, a "The account is already in use" message pop-up may appear. You can try the following steps:
1. Finish the tutorial using the guest account.
2. Sign out by going to Menu (☰) > Settings > Account.
3. The problem will be resolved when you reconnect the game with the linked account.