I have been able to use to be in my translations before the verb before. Why is "What you are describing is impossible" not accepted here? I have not reported it yet because I don't know if the translation is really wrong or not.
I think the simple answer is that Duolingo favours more literal translations and considers yours a better fit for "O que você está descrevendo é impossível" rather than the one given.
I decided to please the owl, but agree that "What you are describing is impossible" would be a much better translation. I'll report it. July 2017.