Game Center log-in window never appearing

Question

The problem here is that the player is not getting authenticated and I'm not getting the log-in window for game center so I really need to get this fixed as soon as possible.

func authenticateLocalPlayer() {

    var localPLayer = GKLocalPlayer.localPlayer()

    localPLayer.authenticateHandler = {(viewController : UIViewController!, error : NSError!) -> Void in
        if viewController != nil {
            self.presentViewController(viewController, animated: true, completion: nil)
        } else {
            if localPLayer.authenticated {
                self.gameCenterEnabled = true

                localPLayer.loadDefaultLeaderboardIdentifierWithCompletionHandler({ (leaderboardIdentifier : String!, error : NSError!) -> Void in
                    if error != nil {
                        println(error.localizedDescription)
                    } else {
                        self.leaderboardIdentifier = leaderboardIdentifier
                    }
                })

            } else {
                self.gameCenterEnabled = false
            }
        }
    }

}