。
She committed a serious blunder.
Literal
She [topic-は] serious blunder [object-を] committed.
失策を犯す (to commit a blunder) is a formal, weighty expression — 犯す (おかす) means to commit (a crime, an error, a sin), and pairing it with 失策 (strategic error/blunder) gives the sentence a tone of gravity. 重大な (serious/grave) as a na-adjective emphasizes the severity. This kind of phrasing appears in news reports, formal narratives, or when describing consequential mistakes rather than everyday slip-ups.