"I am drinking coffee with milk."
Translation:Я пью кофе с молоком.
I don't think this is a mistake. I think it's pretty common on Duolingo: I think you're expected to learn this as a whole expression before you can break it down into constituents. E.g. in the Irish course, I've encountered 'táim i mo chonaí' lots of times already, but haven't seen this construction explained yet.