If you haven't talked to her, she might think you're some stranger. Trust me, if I hadn't talked to my crush and I didn't know who they were sending me a request, I wouldn't accept it. Is there any way you could tell her you like her or somehow get to know her and talk to her without be friends with her on Facebook? Don't worry, she just doesn't know who you are. Girls are a LITTLE more cautious than boys. Maybe try talking to her. You could just ask her a question maybe, or start a conversation by something small. Really hope I helped!