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.
Thank you, Marcy. My boyfriend is Mexican and uses the redundancy more than not.
It was marked wrong for the lack of hermano, had you put Hermano, nosotros vamos a caminar it would have been correct. If that is what you put, then it is an error on their part.