Hello, as the friend above said before you have to downgrade your ETS2 version to the older version if you want to play TruckersMP. This happens only when SCS Software releases an another update to their game to fix bugs, add some little stuff etc. After an update, TruckersMP developers have to make sure everything works well with the new version. So, that takes a little bit of time (~3 days). You can wait for a new update of TruckersMP or you can downgrade your ETS2 version as I said. If you don't want to wait and play, follow these steps:
If you can't see the picture for some reason, or you don't have 6 seconds to watch those steps all I did was:
Go to your Steam library.
Find Euro Truck Simulator 2 and right click on it.
Select Properties and select Betas tab.
Click on the menu there and select the supported version (it's temporary_1_36 for now).
Close the window and let the game update itself. Then you can start playing TruckersMP again.
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EGGSY_DEV's post in Unsupported Version was marked as the answer