Translation:They are without us, and we are without them.
We have no report for your answer, so we cannot tell you why it was rejected. Duo generally ignores punctuation, so either there was some error in your answer or you ran into Duo's intermittent grading bug. It's best to use the Report button when you have a question like this, because that sends your actual answer into the system so we can see it and better answer your question.
The Z in "bez" is indeed pronounced differently -- as either "z" or "s" -- depending on the initial letter(s) of the word that follows it.
At this point in my learning adventure, I couldn't possibly give you the rules for this, but I can tell you that the voiced-to-voiceless or voiceless-to-voiced change doesn't happen only with Z. (Great news, right?)