@arlett_ I guess I tend to say 'ㄹ 수 없다' a little more when I can't do something even though I have the ability. It's more likely for me to say 'ㄹ 수가 없다' to put an emphasis on it. But that doesn't mean you can't say 못 하다 in the same situation. You can use them interchangeably. However, if I mean to say "I haven't learned it and don't know how to do it", for example, "I can't swim", probably I would say "저는 수영을 못 해요". It's just a simpler way to say it.
I know some people say there are differences in usage. Maybe I'm missing something. But I really don't see it right now. I'd say they are practically the same.
Additionally, if someone said, "전 수영을 못해요", that would mean they're not a good swimmer. The spacing makes the difference in meaning (be not good at it) but even a lot of Koreans don't differentiate them and there's no difference in pronunciation. So you should figure out what they mean from the context.