This Epic account was previously linked to a different external account it can only be relinked

Their website only has one answer on the topic, and it has nothing to do with the question.

If you contact their customer support, they simply respond with a copy + paste template, of the same message, regardless of my response. After the fourth time,I lost all hope. This is a prime example of why you should not outsource your customer service — it is the first contact your customers have with your company, and if it is a bad experience, they’ll remember. I played PUBG for months instead of this game for this simple reason.

How to unlink your account

Problem: You cannot link your Xbox live account if someone already has it connected, unless you log in to their Epic Games account.

Solution: Log into their account. Here’s how.

On your PC, open the Epic Games launcher. Log-in with your Xbox Live credentials. In the top-right corner of the launcher, you’ll see the words Manage Your Account. Click that, and it will open the Epic Games website from your browser.

At that point, you’ll be logged in as the individual who has your Xbox Live account linked. From there, you can unlink your account.

-----------------------
@DaveVoyles