If I understand this correctly, when you launch the TMP launcher and then click ETS2, it doesn't open TruckersMP?
Try these solutions:
Is Steam running in the background? If Steam isn't running in the background, make sure you open Steam first before opening TruckersMP. This is because if you try to launch the game, it will open Steam but then produce an error message from Steam's end. It will not launch properly.
Is TruckersMP installed correctly? If unsure or in doubt, I suggest you download the launcher again (Uninstall isn't really necessary but you can do it if you want). Make sure you get the correct file paths!