"Is it a state?"
Estne is used to form yes/no questions, where it must be the sentence's first word. Est is the usual word for "is" in a statement. One might say estne is like "is (it)," while est is like "(it) is." Same with every other verb: add a -ne and put it at the start of the sentence to form a question whose answer may simply be yes or no.
I also tested it and it was rejected. When I clicked on "it" Duo offered "ille" as a possible translation in the last line. Perhaps we would have to use ille/illa/illud and rephrase the question in our heads to ask, "Is that a state?" instead of thinking of it as it/id? Otherwise if id would be proper as well, someone should report it.