This is because he already has an account, so he can't register a new account (that is also against TruckersMP rules to have double accounts.)
The reason he can't register is since he's account is banned until 27 Oct 13:26
He has to appeal his ban in order to play/access TruckersMP again.
Normally, he can access the forums when he logs into his main acccount on truckersmp.com and then clicking forums. But he may receive this error because he is currently banned. (I'm not so sure about that but I think this is the reason.)
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RileyNeko's post in My Friend can't connect his Steam profile with the Multiplayer's was marked as the answer