This error message means that your console is already attached to an Epic Games account. This may have happened the first time that you logged into Fortnite on your console, and you opted not to connect to an existing account.
Follow these steps to find the account your console is connected to:
- Open an incognito tab and go to the Epic Games Account Connections website: https://www.epicgames.com/account/connections
- Select the console icon you want to connect
- Log in using your console login information