How to Fix PSN/XBL "account is already linked to an epic account."
Epic Games recommends creating a new Epic account for each platform from which you play: PC, Xbox, and Playstation. When you played on console, Fortnite created a temporary Epic account for you based on your Xbox Live or PSN credentials, and signing in to the Fortnite Website with your XBL/PSN credentials will allow you to promote it to a full Epic account.
Do not use the credentials for any existing Epic account. If you get an "account already linked" error message, you are most likely trying to use an email address or display name that is already in use.
If you have previously linked an Epic account to multiple platforms, you will want to contact Epic Support to properly move your linked accounts over without deleting your progress.
Once you've created your new Epic account (or promoted your console account to a full Epic account), you can return to Fortnite Scout and view your console stats with your new Epic username.
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