Steam API is currently reporting outdated information to our servers, therefore some users can not register in TruckersMP. We made some extra steps to ensure API stability, however, we were mostly unsuccessful. We have communicated the issue to the Steam Support team and waiting for their assistance.
If you are experiencing issues with registering or linking a game, please try again later (we suggest to wait for an hour or more after a failed attempt). Many apologies for the inconvenience.