First, check that the file has the scs extension. If not, it probably needs to be unpacked.
How to enable the visibility of extensions in windows. (The link will open a new tab)
If you checked this step, now check if the mod with the scs extension has been pasted into the mods folder in Documents/EuroTruckSimulator2/mod.
Now check if the mod has been activated. To do this, start the game and check if the mod is on the right in the mod manager table shown. If you don’t know how and where to activate the mod, watch this video (The link will open a new tab).
Maybe you just don’t know how to use a mod? Find it on the website and see the manual. Most mods come with a short description or video.
If all steps have failed, go to the contact tab or write a comment and write as much details as possible.