This is a polite and humble apology for not being able to help someone. "お力になれず" (o-chikara ni narezu) literally means "not being able to become your strength/help." It's used to express regret when you cannot provide the assistance or support that was requested or needed.

相手を助けられなかったり、役に立てなかったりした際に、丁寧に謝罪する言葉です。