"My všichni jednou umřeme" is not possible?
It is, if you have a special reason to ephasize "we".
It is a correct sentence, but I wouldn't accept it here. I would use "my všichni" where English has "all of us", at least on Duolingo with the Golden rule.
why is ' Všichni umřeme jednou?' not acceptable?
If you want to say that we die just once and not twice... No, it is not presently accepted. It would be once, not someday.