。
I didn't like the ending, you know.
Literal
I [topic-は] ending [subject-が] not-took-a-liking [reflective-な].
気に入らなかった is the past negative of 気に入る — 'didn't take a liking to.' The final な is a sentence-ending particle expressing personal reflection or mild emphasis (more masculine and casual than ね). Common when expressing personal opinions or emotions about a recently consumed work.