Translation:Those who have not received the note must go back to work.
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.
here the algorithm has had extensive training. I gave a comparable answer with synonymous words and relative tenses, and it had a good library to compare it to and accepted:" those that didn't receive the announcement should return to work" is my answer and it was accepted due to this training of the algorithm.