I was going to write "en una hora" but I decided to check by hovering over "an hour" and it indicated "por hora" so I wrote that. Shouldn't have trusted Duo and gone with my instinct since it struck me down. One way to learn I guess.
They are close but slightly different meanings/semantics. 'Necesitamos' means 'we need' where 'Tenemos que' means 'we have to'. Slightly different in usage due to that. If your answer was rejected my guess would be because of that distinction?
i do realize that some rule says to put "que" in between the "we have" and the "to be" but I still don't understand the reason. I thought "que" translated to "what" but even that assumption appears to be wrong. Que estupido! Que demonios!