so there are those two grammatical uhh... thingies in Korean language.
못 X (which was already introduced before) and Xㄹ/을 수 없다 (Xㄹ/을수 없다 in North Korea, with no space in front of 수).
Both of them imply that something cannot be done, that a sentence subject cannot do something. There is a slight difference, though.
못 X means that X cannot be done because of some external causes we can't do anything about. For example: 너무 커서 저기에 못 들어간다 - because I'm too big, I cannot enter there. 입이 없어서 소리를 못 지른다 - because I have no mouth, I cannot scream.
Xㄹ/을 수 없다 means that something cannot be done because of some causes too. For example: 너무 뜨거워서 그 고기를 먹을 수 없다 (먹을수 없다 in NK) - because the meat is too hot, I cannot eat it.
However, Xㄹ/을 수 없다 can also mean that something cannot be done because a person simply don't know how to do that thing. Example: 쓸 수 없다 - I cannot write. 자전거를 탈 수 없다 - I cannot ride a bike (쓸수 없다 and 탈수 없다 in NK).
I'm not a native speaker, so please correct me if I wrote something wrong