I can't add someone as a friend.

You may not be able to add someone as a friend if:
They set their privacy settings to get friend requests only from friends of friends. Ask them to send you a request instead.
You already sent them a friend request. Give them some time to respond to the request.
You already have 5,000 friends. If you need to connect with more than 5,000 people, you can change your personal account to a Facebook Page.
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.