You may receive this error if you do not run Steam as administrator and/or if they are any lack of permissions to run correctly. A solution would be to run Steam and our launcher as administrator:

• Desktop> Steam> Right click> Properties> Compatibility> Run this program as administrator
• Desktop> TruckersMP> Right click> Properties> Compatibility> Run this program as administrator

This way, this problem should be solved.

With the old launcher, this problem can also occur if you try to start TruckersMP when Steam is still closed. Please make sure Steam is already running in the background if you start TruckersMP. With the new launcher, the game will launch in singleplayer mode when Steam is closed.