Translation:Those who have not received the note must go back to work.
I'm a native English speaker...While 'who' is more commonly used, the relative pronoun for humans may also be 'that.' Right?
I am native speaker too and I use ´that´ for people sometimes. I think ´that´is quite informal. ´She is the girl who brings me my coffee every morning´ sounds more formal than ´she is the girl that brings me my coffee every morning´. I guess in duolingo´s sentence they are going for a more formal attitude. What confused me more was ´communique´, I have never heard that word before.
I'm not a native english speake, and I'd like to know if I could use 'shall' instead of 'should' in this case.
"Shall" is used often in BrE in the first person singular & plural as a substitute for "will". Years ago, "shall" in the second and third person was associated with an order or strong request. In AmE, "shall" is limited to suggestions and requests in the first person.
I would use "should" rather than "shall".