A new version of ETS2/ATS has been released that's not yet supported by TruckersMP, so in order to play in multiplayer you need to downgrade the games.
These are the versions you should install in order for multiplayer to work:
temporary_1_22 - 1.22.x in STEAM > Library > Euro Truck Simulator 2 > Right Click > Properties > BETAS
or
temporary_1_1 - 1.1.x in STEAM > Library > American Truck Simulator > Right Click > Properties > BETAS
If you had a different version before, please let the game update on Steam before trying to start the game using the TruckersMP shortcut.