"이것밖에 안 되는 놈이라 미안해" "I am sorry that this is all I have"

놈이 is like "man" 되다 is like "to be" So it might be something like "I'm not a man who can be outside of this/more than I am now"

이것밖에 안 되는 놈이라 = 이것밖에 안 되는 놈이라서 = (lit.) Because I can't be/do any more than this. = Because this is the best I could ever be/do / this is how far I could go. 이것밖에 안 되는 놈이라(서) is a self-humbling phrase meaning that one can't be any more that 이것 (probably refers to some incident that revealed the person's limitations). It could be referring the person's capabilities (as in the translation you quoted)or their integrity as a person. I would lean to the latter because for capabilities, we tend to say 이것밖에 못한다 or 내 능력 밖이라서, or something similar. - 되다 = become / work. Here it means how one is, i.e. one's make-up or potential as a person. (e.g. 그 사람은 됐다 = He's a good person. 사람이 못됐다 = He's a bad person) - 놈 = slightly derogatory (sometimes afectionate too) word for a person or animal. - 놈이라서 = 놈 + -이라서 (이다 + -라서, a special form of -아/어서(reason) for 이다/아니다). e.g. - 내 힘으로 안 돼서 도와달라고 했다. - 내 능력으로는 안 되는 일이라서 도움을 구했다.