I was annoyed by a different answer being marked as wrong... I said "Want a cup of coffee?" and it said that was wrong, but "You want a cup of coffee?" would be correct. In English the you is implied in a scenario like this, so those 2 sentences mean exactly the same thing, right?
I suppose it is implied, but, as in many other cases, Duolingo seems to prefer the most direct translation, barring any slang and situation-based answers. Technically, the "Want" in your sentence can mean "I want," "they want," "we want," etc. That's how the cookie crumbles, unfortunately.
When you use "to do" as the main verb without any other extra ones, it is «fare». Other than that, Italian and other Romance languages (I don't think even German) never have an equivalent for "to do" as an auxiliary verb. So this sentence is literally "Want you a cup of coffee?"