Yes, you must always provide context.

You must provide the whole sentence to get the correct answer. 못한 게 is a colloquial form of "못한 것이" 못한 것이 is composed as 못하(다) + (으)ㄴ + 것+이 못하다 means "not be able to do something" or "inferior to" (으)ㄴ is a particle used for translating a verb into an adjective at past tense. 것 means "thing, matter" 이 is a subject particle 못한 것 means "something which someone could not do" or "something which someone was inferior to somebody" present tense : 못하는 것 future tense : 못할 것 ex) 내가 너에게 말 못한게 있어 There's something I didn't tell you. 좀더 오래 있으면서 프랑스어를 배우지 못한 게 아쉽네요. Too bad you didn't stay a bit longer and learn to speak French.

What's the rest of the sentence?