You are correct; I note this as an English-language copyeditor. In English, unless this phrase appears in a markedly colloquial (non-grammatical) context, the "to" of "not able to" requires a completing verb: "not able to sing," "not able to dance," etc. But the sentence, "No, they are not able," is grammatically acceptable, as is, "They cannot."
There are lots of things Duolingo should accept but doesn't. It's one of its annoying weaknesses, but given that we're not paying for it, I suppose such things should be forgivable. Just click the report a problem tag and let them know next time. If enough of us do so, they'll eventually get around to editing it.