。
She plays piano without sheet music.
Literal
She [topic-は] sheet-music [without-なしに] piano [object-を] plays.
なしに ('without') is a compact adverbial phrase — 無し ('lacking, without') + に — used to negate the presence of something during an action. A near-synonym is なしで, with subtle nuance: なしに leans toward 'in the absence of' while なしで leans toward 'without using.' The talent here — playing without sheet music — is sometimes called 暗譜 (あんぷ, 'memorizing the score') in classical music circles. 弾く is the standard verb for playing string and keyboard instruments (piano, guitar, violin) — distinct from 吹く ('blow') for wind instruments and 叩く ('strike') for percussion.