"He is a man and I am a boy."
Translation:On jest mężczyzną, a ja jestem chłopcem.
That should be accepted, maybe that's just a bug that it wasn't this time.
Actually, "i" was accepted but I deleted it, it doesn't suit the sentence. There are some situations when both work (and have a bit different meanings), but I don't think that it works here. After all, if X is a man and Y is a boy, that surely is contrast.