Swift. Creating a google native ad in SpriteKit

Question

I'm trying to create a google native ad in my SpriteKit game but when I run my code nothing happens, any idea why?

nativeAd = GADNativeExpressAdView(frame: CGRect(x: 0, y: 0, width: self.frame.width / 750 * 600, height: self.frame.width / 750 * 400))
nativeAd.adUnitID = "\(MyUnitId)"
nativeAd.rootViewController = self.view?.window?.rootViewController

let request = GADRequest()
nativeAd.loadRequest(request)

self.view?.addSubview(nativeAd)