。
I don't like it.
Literal
I [topic-は] it [subject-が] not-to-liking.
気に入る is a fixed expression literally meaning 'to enter one's spirit/heart' — the idiomatic way to say 'to like, to be pleased with.' Its negative form 気に入らない directly negates the verb 入る. The liked thing is marked with が, not を, because the grammatical subject of 入る is the thing entering.