。
She got out of the taxi at the department store.
Literal
She [topic-は] department-store [at-で] taxi [object-を] got-off.
降りる ('get down, get off') takes its vehicle with を — the same を that marks the path or starting point of separation rather than a direct object. Compare 飛行機を降りる ('get off the plane'), バスを降りる ('get off the bus'), 階段を降りる ('go down the stairs'). The で on デパート marks the location where the disembarking happens, like every other point-of-action で. Both デパート (clipped from 'department store') and タクシー are loanwords; the casual abbreviation デパート is far more common in everyday speech than the full 百貨店 (hyakkaten), which survives in formal/written contexts.