The `UIFont` constructor is returning an optional (`UIFont?`) which you must unwrap to use.  Add `!` if you're sure you have a valid font name:

**Swift 4:**

    navigationController?.navigationBar.titleTextAttributes = [NSAttributedStringKey.font: UIFont(name: "HelveticaNeue-Light", size: 19)!]
   

**Swift 3:**

    navigationController?.navigationBar.titleTextAttributes = [NSFontAttributeName: UIFont(name: "HelveticaNeue-Light", size: 19)!]