Hello T_Eighteen Firstly, i suggest you verify the integrity of game cache. To do this open steam --> Right click Euro Truck Simulator 2 --> Click properties --> On the top bar click Local Files --> Then Press "Verify Integrity of game cache".
What this does is replace and broken or missing files for that game.
Secondly, ensure all DLC for that game is ticked, For example:
use the scroll bar if needed to check all your game's DLC is Installed, if not, check the box next to it, it then should download automatically.
Regards