。
She gave it her all but failed.
Literal
She [topic-は] with-all-one's-might did [but-が] failed.
Clause-final が works as a soft 'but,' contrasting effort with outcome — a frequent move in Japanese narration where the speaker wants to acknowledge the attempt before stating the disappointing result. やる is the casual all-purpose verb covering 'do, perform, attempt' — interchangeable with する in everyday speech but with a slightly more physical, hands-on flavor. Pairing 一生懸命 + 失敗した frames the failure sympathetically: she didn't fail through laziness.