It is informal to omit a pronoun :)
In questions like this one it sometimes may even sound rude!

Incomplete sentences with omitted pronouns are not in the course.

Yep, totally correct, very spoken (if you speak like this to Ukrainians, they will be impressed), but not accepted here because 1) there are no rules when you're allowed to skip a pronoun and when not, it's highly colloquial and just may or may not "sound weird", so it would be hard to teach 2) we are teaching "textbook" Ukrainian and omitting spoken things (cause that would be way too much to add)