"He is usually angry."
Why is 一般 in some exercises "usually", but here it is not accepted? And isn't 经常 "often"?
经常 is ambiguous, and has either translation depending on the context: “usually” or “often”.
As a standalone sentence, both 他一般很生气 and 他经常很生气 sound a bit weird to my native ears. 经常很生气 is not how we usually state the status of someone. There should be a context, e.g. 每当我讲笑话的时候，他一般很生气。Every time I tell a joke, he is usually angry.
一般 and 经常 can both be translated in English "usually." 他经常很生气 means that the person gets angry easily but somehow the status of anger will end. but if you say 他一般很生气 means that he has a constant status of being angry and seems there's not an end which is kind of weird.
besides, i don't think this chinese translation is good enough. "他经常生气。" is better then "他经常很生气。" . "很" means "very" in English, but I don't see the hint of "very" in the original english sentence. And "他经常很生气。" sounds weird in Chinese too...
很 can also be a stub to link the subject with the adjective, not necessarily means “very”. This course is quite pedantic about this function.
I also think the translation provided sounds weird on its own, and then it should be 他经常都在生气. What you think is better, 他经常生气, would rather mean “he often gets angry about some matters”. Because 生气 is a verb here, while 很生气 is a status (or adjective).
We use 一般 to describe the habit instead of emotion. It's better translate "be used to". I'm used to get up early. 我一般很早起 Also you can replace 通常、經常. But if you use 一般 to describe the status is inappropriate. Actually, this sentence isn't like the native, I will say 他經常生氣 or 他很常生氣 or 他常常生氣。