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이 못 돼

isn't suitable to be

→ 이
phrase Uncommon #9,104
isn't suitable to be 이 marks the preceding noun as the subject; 못 돼 expresses that it is not suitable or possible to be something.
KO: 여기는 아이를 키울 곳이 못 돼.
EN: This isn’t a suitable place to raise children.

자네 아버지는 그럴 인물이 못 돼

Your father isn't the kind of person who would do that.

— 히든 (2005)

당신은 절대 부사장이 못 돼

You'll never become vice president.

— 400번의 구타 (1959)