Fatal error: init(coder:) has not been implemented. Where is the error when running

Because you have overridden the designated initializer for SKScene with your override init(size: CGSize) method and therefore have to implement this required init?(coder aDecoder: NSCoder) method to allow the custom SKScene class (your GameScene class), object serialization.

This older article might help explain this for you:

What exactly is init coder aDecoder?

If you are trying to initialize the gameArea variable, why not just try something like setting gameArea in an override of sceneDidLoad()? (really awful code below):

class GameScene: SKScene {
    let player = SKSpriteNode(imageNamed: "playerShip")
    let bulletSound = SKAction.playSoundFileNamed("bulletSound.wav", waitForCompletion: false)

    var gameArea: CGRect?

    override func sceneDidLoad() {
        let maxAspectRatio: CGFloat = 16.0/9.0
        let playableWidth = size.height/maxAspectRatio
        let margin = (size.width - playableWidth)/2

        gameArea = CGRect(x: margin, y: 0, width: playableWidth, height: size.height)
    }

    <<more of your code>>
}

and eliminate the override init(size: CGSize)

Like oaccamsrazor states, if you add an initializer to your class, you also have to implement any required initializers in the superclass. The fact that you're hitting init?(coder aDecoder: NSCoder) indicates init(size: ...) isn't getting called, likely because you're creating your scene from an .sks file.

init?(coder aDecoder: NSCoder) will get called when creating your scene from an .sks file, so you can't have fatalError() there. Basically, just actually implement your initializer if you have anything to initialize. Otherwise, delete all your initializers and the requirement to have init?(coder aDecoder: NSCoder) will disappear.