ProMods

I have the same issue

My mods folder conatins the following files:

promods-assets-v251.scs
promods-def-st-v251.zip
promods-map-v251.scs
promods-me-assets-v251.scs
promods-me-defmap-v251.scs
promods-media-v251.scs
promods-model1-v251.scs
promods-model2-v251.scs
promods-model3-v251.scs

It works perfectly in Single Player ETS2 but not in TruckersMP unfortunately

The def file needs to be .scs too, not .zip.
Your browser is misconfigured since the def file always comes as .scs, so try download using a different browser.