"You drink a hot chocolate."
Translation:Tu bois un chocolat chaud.
Technically, you're correct when saying that "Tu" isn't necessary. You could use "Vous," but that would be formal. None the less, "Vous" is still correct. My guess as to why it was counted as incorrect is that Duolingo uses "Vous" in the plural sense, and not really the formal sense. The program doesn't really identify/pick up on formal sense too well, in my experiences.