Assuming that you were responding to someone else's “Friend Request” they sent to you, as your pic shows you are in the “Received Requests” screen.
By the time you got around to accepting that person's Friend Request, that person who requested you had since filled up their Friends List to Max, sometime after having sent you their Request.
So not quite as “simple” as normal situations where you are the one trying to send a Request to someone else who's list is full.
Comments
but may I suggest you to take a good look at the image OP posted
it clearly states they have 31 friends in game and it still says friend list full when the limit is 50
Your friend list is full
By the time you got around to accepting that person's Friend Request, that person who requested you had since filled up their Friends List to Max, sometime after having sent you their Request.
So not quite as “simple” as normal situations where you are the one trying to send a Request to someone else who's list is full.