The word by itself and the discussion picture suggest "missing screws" as the correct interpretation.

"Missing XXX" is the normal way of referring to something that people forgot to do or add in.

But it's odd that the device can detect missing screws. Also, the first two major errors are "missing screws" and "incorrectly threaded screws." Wouldn't screws that were placed but not tightened fit in between those??

If the tightening was forgotten, I think the phrase would just refer to the result "loose screws" or "untightened screws" rather than to the action or the person who made the mistake.

ねじそのものを忘れる意味だとは確かに考えにくいし、文章にあげられた二つの大きなミスはおそらくねじ締め忘れとねじ締め違いでしょう。特に「検出」ができるという点から考えたらそうです。しかし、写真があることとわざと曖昧な表現を使う説得文章であることからみて「ねじ入れ忘れまで直しちゃう！この道具」って感じになってもいいかな。

Anyway, "missing screws" or "loose screws." Or use both...