Translation:You cannot go home yet.
This course definitely needs a textbook on the side. I work my way through Duolingo and use Genki next to it. Practicing on DL experience makes the Genki chapters very easy, and when I encounter something new in DL I can get an in-depth explanation in Genki.
That said, I think the course could still add some more grammatical info. There are a small handful that do and it makes it way easier to get started.
That is not necessarily an imperative/command form. It's a simple statement that you could be saying to someone else (or yourself, though you might not say it so politely to yourself). You could say to a coworker: "I mustn't go home yet as I still have work to do" or "I must not eat this chocolate since I'm on a diet". Also, the particle wa is written as は.