。
She didn't attend the lecture.
Literal
She [topic-は] lecture [at-に] did-not-attend.
出席する ('to attend, to be present at') takes に for the event attended — 講義に出席する, 会議に出席する, 結婚式に出席する. The preposition is に because attendance is treated as 'arriving at' the event. Negative past 出席しなかった is the plain form; the polite version would be 出席しませんでした. 講義 ('lecture') is the standard term for university-style lectures, distinct from 授業 ('class, lesson') used in primary and secondary education.