Why I Can't Send A Friend Request To Someone?

You may not be able to add someone as a friend if:

• They set their privacy settings to only get friend requests from friends of friends.
• You've already sent them a friend request. Give them some time to take action on the friend request.
• You've blocked them. To send a friend request to someone you've blocked, you need to unblock them first.

Note: If requests you send go unanswered or are reported as unwelcome, you may be blocked from sending friend requests.

Other answer:

Maybe it's because of her privacy sitting, she probably allows only "friends of a friend" to add her. It's her who have to set her friending setting to "everyone", not you. You need add friend with her friends first or send her message and ask her send friend request to you.

Read more: How To Edit Who Can Send Friend Request?