。
She can't play the piano.
Literal
She [topic-は] piano [subject-が] can't-play.
Two ways to express 'can/can't' in Japanese: ~ことができる (the slightly formal nominalized version) and the potential form (~れる/~られる, here 弾ける). When the potential form is used, the object of the underlying verb is typically marked with が rather than を — ピアノが弾ける rather than ピアノを弾ける. Either pattern is acceptable, but が is the textbook recommendation and reflects the older logic that potentials are stative descriptions of the experiencer.