"You need coffee!"
This is genitive, as „potrzebować” belongs to a group of verbs that goes with genitive. Somebody else might give you more detailed info.
Well, that's the same as "You need coffies!" - theoretically correct, but pretty rare. And simply not a translation.
Shouldn't such a sentence need imparative (is this the name in English as well? I mean the case you use to order people.)?
Although technicly possible almost no one uses imperative forms of "potrzebować" like: potrzebuj or potrzebujcie. That's one thing.
This sentence doesn't make use of imperative. This is rather an ovservation/advice. (You look exhausted). You need coffee!