→Make sure your ETS2 is set to "NONE-opt out of all betas" in Properties>BETAS , as shown in picture
→Make sure your TruckersMP Launcher has the correct file path of the game, else it wont even bother to open. To make sure your launcher has the correct file path, Firstly Go in Local files tab and click Browse local files, Copy file path as shown in Explorer, Example image:
→Make sure no program is blocking the opening of Launcher, Example, Outdated RTS Server which MSI Afterburner uses or Outdated EVGA PrecisionX hooks,etc. So, If you have any softwares like those to monitor PC/Game stats, update them and retry.
→Last but not the least, Perform a clean boot, Guide→ https://support.microsoft.com/en-in/help/929135/how-to-perform-a-clean-boot-in-windows
Awaiting for your response.