Are you trying to move a platform account from one Bungie profile to another one, ie an Epic account from one Bungie profile to your current Bungie profile or your Steam account to a different Bungie profile? If you are then the March 25th linking change is why you're getting the error message as they made account links permanent and you cannot unlink accounts from a Bungie.net profile or move them between different profiles anymore. The only exception to the linking rule are Twitch accounts.