Steam API is currently reporting outdated information to our servers, therefore some users can not register in TruckersMP. As the issue is out of our reach, unfortunately, there is nothing we can do to resolve the problem.
If you experience that, please try again later. Apologies for the inconvenience.
TruckersMP does not support the latest game version of Euro Truck Simulator 2 or American Truck Simulator yet.
Check out the following guide on how to downgrade so you can keep playing on multiplayer:
How to downgrade