Fitting with the precedent of how Duolingo handles other idiomatic phrases, that should be accepted so report it if not.
But you should know that when a German says es geht, especially in the context of you asking how they are doing, it means something noncommittal somewhere between "well" and "terribly".
I believe noncommital just means it falls between well and terribly and is liable to change easily. So-so is a phrase in English, and going along with the OP's question, I have rarely but occasionally heard "It goes" or "It's going" as a similarly noncommital response to "How's it going?"
(native English here) I can picture myself nodding my head up and down when saying "It's going all right" and gesturing with a shoulder shrug when saying 'So so" - in both cases I am acknowledging the person's question without going into details (unless I want to). Sorry if I'm too off topic with intent of this discussion thread.
Two pass reply: - In the realm of idioms, if one were to say “It’s fine!” with the enthusiasm of an adolescent reflecting after their first kiss, versus the same reply from a tenured I’m-just-glad-to-be-home employee: same words, different meaning. - From a monotone stance, It’s uber-tastic, It’s great, It’s fine, It’s alright, It’s par, It could be better, It’s …., these relay levels of personal wellbeing without visual or auditory cues. Sumary: Until Duo offers full emersion VR, may be best to think in terms of monotone answers.
Technically, "It's good" would be es gut, while "It's alright" would be es geht's. It's a small difference, and they have the same meaning in a conversation, but I think duolingo marked you as incorrect because gut and geht's are different words. TL'DR: The connotation isn't the important thing, learning different words in German is. (Hope this helped a bit!)