TruckersMP doesn't support the latest game version of American Truck Simulator yet.
Check out the following guide on how to downgrade so you can keep playing on multiplayer: How to downgrade
Before creating a support ticket about your game version not being supported in TruckersMP
SCS released an update for ATS. We do not support the latest version of American Truck Simulator yet. You need to wait until we release a game update in order to play the newest version of ATS in our MP.
Downgrading to an older version is possible. More information can be found here