Technically, since Russian doesn't have articles, a girl should be accepted, I think. However, it's awkward to say "a girl does not have milk" in English. You would rarely ever say it. The fact that you know that she doesn't have juice implies you're talking about a specified girl, otherwise you're either making an assumption that somewhere out there, there exists a girl who does not have juice, or you're speaking generally and suggesting that a characteristic that defines a girl is that she doesn't have juice.
http://www.russianlessons.net/grammar/spelling_rules.php Rule #1 may help explain things a bit
Because when a person says that in English, it's a colloquialism where they're dropping another word. They're saying "The girl does not have a juice [box]" or "The girl does not have a [glass of] juice." Because juice is in a fluid form, you can't grammatically have it in singular form like that.
When we are required to speak into the microphone, it is being marked correct before I have finished the whole sentence (provided I have carefully and correctly pronounced the first few words). This is happening in multiple exercises; not just this one.
On the old version (two days ago), if you said it too fast and got it wrong, it would ask you to "Try speaking more slowly."
The microphone co-ordination to the exercises was much better on the older version (up until two days ago).
сок (sok) [sok] m inan (genitive со́ка, nominative plural со́ки, genitive plural со́ков) "juice; sap; semen": From Proto-Slavic *sokъ ("juice"), from Proto-Balto-Slavic *sakas, from Proto-Indo-European *sokʷos. Cognate with Lithuanian sakai (“resin, tar”), Ancient Greek ὀπός (opós, “juice”), Albanian gjak (“blood”) and Latin sappinus ("a type of fir", via Gaulish).
My bad alvie_x already kindly posted the answer with this link http://www.russianlessons.net/grammar/spelling_rules.php
Russian expresses "to have" in a different way, "У девочки есть" (literally, "To the girl is..." means "The girl has..."
Similarly, "У девочки нет" (literally, "To the girl is not...", means "The girl does not have...."
Hebrew does something similar: "לילדה יש ...." means "The girl has...." and "לילדה אין..." means "The girl doesn't have....."