Google translate says me, that "it's allowed" is "Это разрешено". I think, that "be allowed" and "be possible" are two different things. It is not possible for people to breathe on the moon but it is allowed, because no one has forbidden it.
Isn't "impossible" and "forbidden" the same thing in Russian? Like if you want to say "Smoking is forbidden here", don't you say "Здесь нельзя курить"?
Yes, you can use "нельзя" for both "impossible" and "forbidden". There are special words, too: невозможно = impossible, запрещено = forbidden.
I wrote the same, and also rejected. English is not my first languaje so, any english speaker could explain why is it rejected?