Translation:He is uncertain.
Dear people, I mean this with all respect, but this far into the course, haven't you noticed that Korean is a contextual language? Subject pronouns are optional and everything depends on context. It could be I am uncertain, you are uncertain, he or she is uncertain. If Duolingo only accepts "he is uncertain", then that's very bad. But in real Korean, the subject is often implicit.
. . . or is this a continuation of the last answer's jealous woman thing, the context coming from that?