Facebook friend request sending problem?

[Geek]

I am trying to add one of my friends, we have 11 mutual friends , but whenever i am pressing the add friend button , this msg is showing -"Friend requests are for people you know, like classmates, friends, family and coworkers.To learn more, please visit the Help Center.

If you think you're seeing this by mistake, please let us know."

please tell me why this is happening and what to do, and the let us know link is not being helpful
I have never sent them any friend request before..

[matthew herron]

you can contact facebook at contact.facebook.com use the email or it might because your friends privacy settings doesn't allow people to send him/her a friend request you could still message her and ask her if she can send a friend request

[Daniel]

Do you Know Them ? Its Possible if you Sent them a Request in the Past They May Have Marked Down That They Dont Know you So it Wont Let you Send them a Request if you Do Know them And if you have Not Sent Them a Request Before Than its a Error With Facebook Please Contact Them at the Help Center and Report the Problem To Them

[Karen]

You should ask your friend to send you friend request through massage conversation.

or ask to help center for this issue.