Couple of days ago I was in the same situation facebook was not allowing me to sent friend request to anyone on facebook continue getting the same error was quite annoying. It says ‘Do you know this user personally? To prevent misuse of facebook this request can’t be sent’. So there was no other option to contact that user except facebook message to whom I was about to sent a friend request.
Well now a day’s facebook has tightened their privacy because of high spamming. If you too were figuring the same error while adding someone of facebook then this could be a helpful article for you.
I did lot other ways to fix this error but nothing worked I thought the error is due to a lot of friend requests from me went unanswered. So I tried editing my friends and removing pending friend request but unfortunately it doesn’t work for me.
After Google for hours I found the solution to fix that error. To fix it you need to go to your facebook profile now change your gender to female (if you’re male or change it to male if you’re female) and save. Now send a friend request to the same girl you were sending before and it will work. After your request is sent, go back to your profile and switch your gender back.
Apparently, it has something to do with spam protection on facebook.