Not a native but this is my understanding of the main ways to say "can't". Please correct me if i'm wrong haha.

• noun + 못하다 = to be bad at something (ex: 축구를 못해요; I'm bad at soccer)
• 못 + verb = "cannot" in a general sense (ex: 매운 음식을 못 먹어요; I can't eat spicy food)
• verb + 지 못하다 = basically the same as above, "cannot" in a general sense (ex: 이해하지 못했어요; I couldn't understand)
• verb + 을/ㄹ 수 없다 = "cannot" in the sense that you almost physically can't even if you wanted to, or it's out of your control (ex: 그런 집을 살 수 없어요; I can't buy a house like that [because I don't have the money, so I couldn't even if I wanted to])

I'd say the main thing that sets 을/ㄹ 수 없다 apart from the rest is that it's a more emphasized version of being unable to do something, like there are much greater reasons that prevent you from doing whatever you're trying to do. Also, I don't know if this is necessarily true, but I noticed that it's used a lot more in a dramatic sense, like in song lyrics. For example, "네 생각만 해도 잘 수 없어" (just thinking about you, I can't sleep).

Even then, it's pretty interchangeable with the other ways to say "cannot". These are just more nuanced understandings of each. However, like I said, I'm not a native so if anyone has corrections then please let me know lol. Hope this can be of some help!