I would argue that, to a native English speaker, that sounds very unnatural. That might be why it's not accepted. Remember: literally translating word-for-word isn't how language works.
I suppose it would actually mean. What is that you are drinking? (Surprise and shock) Could it be milk? Or its like saying, I can't believe it are you drinking milk? Meaning and exact word for word translation don't always correlate.
I have trouble in transliterating.. When I see the azbuka sentence and I have the "chto eto ty pesh moloko" it looks identical to me... Is it the word "pesh / pyosh" - should I have been more phonetic here? Or is it elsewhere?
I did succeeded with apostrophes "chto eto ty p'esh' moloko" ? However it still sees the sentence contains some typos. So what would be the perfect transliteration?