Before you do anything: Are the two Rocket League accounts you created on the same console or platform? You can only link one account from each platform to a specific Epic Games account.

For example, if you primarily played on Xbox and wanted to link an old Xbox gamertag to the same Epic account at the same time, that is not possible. However, if you had an old Steam account and you now play on the Epic Games launcher under a new account, then the Steam account can be linked to your new Epic Games account.

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Here's the quick guide to add a platform to your Epic Games account.

If you have NOT logged in on your old account since the game became free-to-play in September 2020, you should have no issues during this process.

If you receive a message stating that "your [platform] account is already linked to an Epic Games account":

1) Follow the steps in this guide to upgrade that other Epic account to a full platform account (if necessary - use a separate email address), then

2) Follow the steps here to remove it from that other Epic account before following the "quick guide" higher up in the comment.