Translation:When his mood is bad, he cannot express his emotions.
If you mean it accepted "humor" in an English translation of the Esperanto, it is a valid synonym of "mood" and a valid translation. Although, despite that, I wouldn't use it here (so maybe it isn't quite so valid).
2 A mood or state of mind:
her good humor vanished
Has anyone tried writing "When he's in a bad mood..."? I haven't tried it yet, but I feel like that's much more idiomatic in English. I'm pretty surprised no one else has asked or mentioned this, to be honest.
This was marked wrong, "when his mood is bad he is unable to express his emotions"??