At first you need to make sure the game is fully installed in your computer: Right click ETS2 game- properties, Local files, Verify integrity of game files. Steam will automatic check the files.
Then check the TMP launcher ETS game directory, the path supposed be look like "C:\Program Files (x86)\Steam\steamapps\common\Euro Truck Simulator 2".
If the game directory wrong MP mode will not working. find the correct path : Right click ETS2 game- properties, Local files, Browse the local file to find the correct path, copy them to the launcher; at last save.
Hope this can help you, have a nice day!