I think its the same since 못 and 수 없어 both mean " i can't" Ex: 한국어를 할수없어요 한국어를 못해요 but: when you add 잘 to 못 and to 수 없어요 like 한국어를 잘못해요 it means that you are not good in korean not that you can't speak it at all. ^^

못해 is a short and easy way of saying 할 수 없어.

못 해요 means "I can't do"I can't do so, it's against my nature You can say 안 돼 (i think u should know this too)to mean "It doesn't work" when컴퓨터가 안 돼. The computer doesn't work. (I can't-ㄹ 수 없습니다 is like "There's no way you(or we)"capacity, way, method, ability". It's formal. 잘 수 없습니다. I can't sleep .고칠 수 없습니다. I can't fix it.