AdMob Banner Ad won't display in Swift SpriteKit game

Question

I am creating a game in GameScene/SKScene using SpriteKit, and I am trying to display a AdMob Banner Ad. Somehow the Banner Ad won't show up.

This is the Banner Ad part of my code that I am currently using.

import SpriteKit
import GameplayKit
import GameKit

// AdMob
import GoogleMobileAds
import UIKit

class GameScene: SKScene, SKPhysicsContactDelegate, CheckboxNodeDelegate, GKGameCenterControllerDelegate, GADFullScreenContentDelegate, GADBannerViewDelegate {

var bannerView: GADBannerView!
var isBannerAdReady = false

override func didMove(to view: SKView) {
    ...
    loadBannerAd()
}

func adView(_ bannerView: GADBannerView, didFailToReceiveAdWithError error: Error) {
    print("Failed to load banner ad: \(error.localizedDescription)")
    isBannerAdReady = false
}

func loadBannerAd() {
    // bannerView = GADBannerView(adSize: kGADAdSizeBanner)
    bannerView = GADBannerView(adSize: GADAdSizeFromCGSize(CGSize(width: 320, height: 50))) // Adjust the size as per your requirements
    bannerView.adUnitID = "ca-app-pub-3940256099942544/2934735716" // Replace with your own ad unit ID
    bannerView.rootViewController = self.view?.window?.rootViewController
    bannerView.delegate = self
    bannerView.load(GADRequest())
        
    // showBannerAd()
}

func showBannerAd() {
    if isBannerAdReady {
            self.view?.addSubview(bannerView)
        isBannerAdReady = false // Set the flag to false after showing the ad
            } else {
        print("No ad ready to be displayed.")
    }
}

// Tells the delegate that a full-screen view will be presented in response to the user clicking on an ad.
    func adViewWillPresentScreen(_ bannerView: GADBannerView) {
        // Pause your game or perform any necessary actions when the ad is clicked
    }

    // Tells the delegate that the full-screen view will be dismissed.
    func adViewWillDismissScreen(_ bannerView: GADBannerView) {
        // Resume your game or perform any necessary actions when the ad is dismissed
    }

    // Tells the delegate that the full-screen view has been dismissed.
    func adViewDidDismissScreen(_ bannerView: GADBannerView) {
        // Load a new banner ad when the previous one is dismissed
        bannerView.load(GADRequest())
    }

    // Tells the delegate that the user click will open another app (such as the App Store), backgrounding the current app.
    func adViewWillLeaveApplication(_ bannerView: GADBannerView) {
        // Handle the user leaving the app, e.g., pause the game
    }

    // Tells the delegate that a click on an ad failed to open a full-screen view.
    func adView(_ bannerView: GADBannerView, didFailToPresentFullScreenContentWithError error: Error) {
        print("Ad did fail to present full screen content.")
    }
    
    func bannerView(_ bannerView: GADBannerView, didFailToReceiveAdWithError error: Error) {
        print("error mess: \(error)")
    }

When running the showBannerAd(), no Ad show up, and I get the message:

No ad ready to be displayed

What am I doing wrong? I added the Ad Unit for over 24 hours ago, and I am using the correct App ID. I am already using interstitial ads in my app that is working perfectly.