[BY DESIGN] Ordering parts missing from computer

This might be just the intentional way it works.

As far as I can see in the tuning files, the restrictions for whether or not it appear include:

1) your sim must be at least level 2 in handiness skill

2) your sim must OWN the computer they are trying to order it from

This means public computers at the library wouldn't work, but computers at your home while your sim bought or at a owned business your sim bought will work. Or possibly if you have the portable laptop and took it to a library, that would work because you own it.

The purchase medicine also used to work this way too, before they removed the "you must own the computer" restriction. You'd have to ask the developers why there's the difference. Maybe because they think you wouldn't normally be away from your home ordering parts to upgrade stuff that isn't at your home? I know now with the new off-the-grid lot trait, you might need this restriction gone, so that you can order parts from a public place.