"Do you take a walk often?"
Translation:Ты часто гуляешь?
So what is the difference between "Ты часто гуляешь?"(which is listed as correct answer) and "Вы гуляете часто?" (which was not accepted).
No, you can just say "Часто гуляешь?", but I wouldn't start a new topic this way. I would rather use this sentence if walking was already mentioned in the conversation somehow.
Somebody tell the Duolingo's team that they need to check the russian answers. I am linving in Russia now and "вы гуляете часто" is also correct. Russian language are less strict with the order of the words.