。
She couldn't fasten the zipper.
Literal
She [topic-は] zipper [object-を] fasten [can-ことができる] was-not-able.
ファスナー is one of those loanwords whose Japanese meaning has narrowed compared to its English source — in Japan ファスナー refers specifically to a zipper, while English 'fastener' covers buttons, clips, latches, and hooks too. The synonym ジッパー ('zipper') also exists but is somewhat less common in everyday Japanese. とめる ('fasten, fix in place, stop') is the transitive partner of とまる; in clothing contexts it covers buttoning, zipping, clipping, and pinning all under one verb. The ~ことができる/ない construction provides the 'can / cannot' frame around the verb, here used in past negative.