Except some places DO say 'the staff' and 'the couple', for example, in the plural sense. I'm from the USA and we (at least I) don't use it in the plural..... but I now live in Australia, and they DO use those and many others in the plural. They'd say "the team ARE", for example, also.
I made a few reports because there seem to be many possible translations that should be accepted and aren't. "The staff work through the weekend." "The staff work over the weekend." "The staff work the weekend." All of these sound natural to me, a native English speaker. Is there any reason why they should not be accepted? (Aside from that Duolingo doesn't recognize it for mechanical reasons, of course.)
In American grade schools [here anyway], "The staff work over the weekend" is taught as incorrect subject-verb agreement regardless of whether it is meant as a collective. I teach English, and this is explained as a single collective needing a singular verb in their text. If there is more than one collective you could say something like, "The staffs work together during the merger." I can't speak for UK English, but the sentence was jarring to read to me.
The staff work the weekend should be an acceptable answer here surely. 'During' is not natural in this sentence for English, i dont think ive ever heard any one say they work during the weekend. On, over, through, at, anything but during, unless på is specifically 'during' anything else would be more natural.