Sorry if this is simplistic, I did search for an answer.
I have a supermoderator who can't send or acknowledge friend requests. I have checked the forum permissions and usergroup permissions and friends are allowed. I have registered users who can use it as I have one as a friend.
If I send her a friend request, she clicks on it and nothing happens. If she tries to send me one, it tells her she doesn't have permission, refresh the page and login before trying again.