"It is a dark night."
Translation:Jest ciemna noc.
It is accepted, it should have worked.
Note that the meaning is different from "Jest ciemna noc", though.
With most sentences, "[This/That/It] is Y" mean exactly the same to a Polish person (seriously, I literally can't understand the difference and it's killing me in the Spanish course which has 3 separate translations for those). But a sentence like "It is a dark night" most probably means that 'right now' we have a dark night. So "it" doesn't really refer to anything, and therefore has no equivalent in the Polish sentence "Jest ciemna noc". Meanwhile, "To [jest/] ciemna noc" meanс the same as "[This/That] is a dark night".
OK, I'm not really sure if at least "This is a dark night" can't be interpreted as 'right now we have a dark night', but that's how I can explain it.
Uff, I somehow managed to not write "dark knight" accidentally even once!