。
Because she can never keep a secret.
Literal
She [topic-は] never secret [object-を] cannot-keep [because-から].
An unusual sentence-final ~から hangs the whole utterance as a reason without supplying the consequence — 'because she can never keep a secret… [something]'. This trailing-から is common in spoken Japanese as a freestanding reason in a conversational exchange ('Why don't you tell her?' — '— because she can never keep a secret'). 守れない is the negative-potential of 守る ('to keep, protect'); 秘密を守る is the standard collocation for keeping a secret.