"No, you do not know her."
Translation:Нет, ты её не знаешь.
Duolingo puts together, in the same thread, discussion about a given phrase in both directions, that is, from Russian to English, and English to Russian. If you are translating from English, you may choose between ты and вы - at least, if you work from the computer. At the phone, they sometimes put the first words fixed, or even the full answer!
The printed question used ты so I completed the sentence for ты. It was marked wrong with the correct solution in red giving the sentence in вы format. If the original printed sentence had been plural I would have completed it in plural but the original was singular ты, which I was not at liberty to change. I think it's an error as the question/answer don't match. I was just flagging it up......