Why is the Nos essential here?
It's not. What was the full text of your answer?
That was it ---- I wrote Bibliothecam visitare volumus and it was marked wrong. If I made a spelling mistake somewhere I can't check it now. Thank you for clarification on the essential point though!
Interesting, here in the forum it shows up in the preferred word order that I was going to argue for. I was given "Nos visitare bibliothecam volumus" as the correct answer.
volumus visitare bibliothecam
That is English syntax, not Latin.