。
She didn't notice the mistake.
Literal
She [topic-は] mistake [target-に] did-not-notice.
気づく ('to notice / to realize') always takes に for what was noticed — never を, even though English would say 'noticed *the* mistake' as a direct object. に here marks the trigger or target of cognitive contact: 'noticing happens to land on this thing.' This に-with-cognition pattern shows up across Japanese: 気がつく, 思いつく, 行きつく — verbs ending in ~つく ('attach to / reach') pair naturally with に. The negative 気づかなかった is in past tense, framing a single past event rather than a habitual non-noticing.