Please follow this solutions:
Open TruckersMP in Administrator Mode.
Install the available updates (only the first run with Ultrasurf), wait for it to finish and Launch button appears.
In order to verify your game cache, go to your 'Library' and find 'Euro Truck Simulator 2'.
Once you find it, 'Right Click' and go to the 'Properties ' tab.
This will open up the properties tab. Go to the third sub-tab that says 'Local Files' where you will be presented with the following options.
Click on the last option to begin the validation process. This might take a few minutes.
Once all the files are validated, if any corrupt instances are found, then Steam will redownload those particular files for you. If not you will be greeted with a box that says no errors.
You have verified your game cache successfully.
"Usually it appears when you try to update the multiplayer launcher with the game open.
First, check if ETS2 process is running. If so, end it and try again. If this doesn't help delete core_ets2mp.dll, re-open launcher and re-download file. Path: C:\ProgramData\TruckersMP"
Press the Windows Key + R (Win+R) on your keyboard
Enter "%programdata%" and press Enter (without the quotes)
Find the folder "TruckersMP" and delete that folder.
Navigate to C:\Program Files\ and delete the folder named "TruckersMP Launcher"
Go to our website and re-download and install the Launcher with admin privileges
Open The Launcher with admin privileges and click "Install Available Updates"
I hope it helps...