Translation:Hermano, vamos a caminar.
I don't understand why "Nosotros vamos a caminar." was marked wrong. I would think it should be an option to an implied "we".
Dúo has been using redundant subject pronouns in a lot is the recent sentences. I'm surprised they would count yours wrong. It's really not needed here, but it's not really wrong to include it. If you want it to be accepted, try the Report button.