Hey there, @Tomstarz.
It seems that you're providing an invalid path for your Euro Truck Simulator 2 game during the launcher installation.
To find your path, please follow below:
1 - Head to your Steam Library, Right-Click on Euro Truck Simulator 2 and go to Properties.
2 - Once in the Properties area, head to the Local Files tab and click on Browse Local Files.
3 - This should open a file explorer, I need you to head to the top of the window to copy and paste the path.
For reference, my path is:
Let me know if this helps!