https://wiki.cartelempire.online/en/Player/Cartel/Enhancements

TL:DR; you either need to complete the first Engineering course or your Capo needs to give you access to the Cartel's one (providing it's unlocked!)

Enhancing weapons is expensive (very expensive) and not something to worry about until end game and you have very high quality items (85%+ at least).

I'm presuming this question is because you've got a mission to enhance - I wouldn't bother with these personally, it's much better to spend the points to reroll and save those precious mats for later down the line.