。
She admitted she had made a mistake.
Literal
She [topic-は] mistake [object-を] committed [thing-こと] [object-を] admitted.
Two structural facts are worth noticing. First, 間違いを犯す ('to commit an error') — 犯す, the same verb used for committing crimes (罪を犯す), gives 'mistake' here a slightly heavier moral weight than the everyday 間違いをする, hinting at consequences. Second, the embedded clause is nominalized with こと: 間違いを犯したこと ('the fact of having committed a mistake') becomes the object of 認めた ('admitted'). こと is the standard 'fact-style' nominalizer for events being acknowledged, recognized, or known — distinct from の, which would feel more sensory or direct. Together they produce a register that's deliberate and slightly formal — the kind of phrasing used in news reports or character descriptions.