You might receive this error message while trying to link your Steam ID to your Runic account here.
In almost all cases, this issue is caused when the user has previously linked their Steam ID to a Runic account. Then, at some point later, the user tries to link their Steam ID to a different Runic account, forgetting that the Steam ID may only be linked to one Runic account.
Try checking for various username possibilities under which you might have created a different Runic account. If you still are unable to locate your old account credentials please refer to the Password Recovery Issues article.