Those errors usually occur when having entered a wrong Euro Truck Simulator 2 / American Truck Simulator directory during the TruckersMP Installation.
For that reason you should uninstall the TruckersMP Launcher and install it once again.

Please follow the installation process as usually, but when the installer asks you for the ETS2 / ATS directory, please follow these steps

1. Go to your Steam library.
2. Right click on ETS2 / ATS.
3. Choose "Properties" and go to the "Local Files" tab.
4. Choose "Browse Local files..." and a folder with gamefiles is going to open.
5. Copy the filepath of this folder and paste it in the Installation window.
6. Complete the installation and try to start TruckersMP again.

If the above Solution does not solve your Problem, please contact our Support Team here. If you need help while creating a Support Ticket, take a look at this article.