iOS Swift集成AdMob测试广告失败：无广告可展示请求错误求助

我的应用无法获取广告，就连官方测试广告ID也不行，当前使用的测试广告ID为ca-app-pub-3940256099942544/2934735716。已经在谷歌搜索相关问题多日，但未找到适配我情况的解决方案，恳请各位帮忙排查，谢谢！

错误信息 #

Error Domain=com.google.admob Code=1 "Request Error: No ad to show." UserInfo={NSLocalizedDescription=Request Error: No ad to show.}

我的代码（GAD SDK版本：10.4.0） #

AppDelegate #

...
GADMobileAds.sharedInstance().start(completionHandler: nil)
GADMobileAds.sharedInstance().requestConfiguration.testDeviceIdentifiers = [ GADSimulatorID ]
...

Controller #

...
AdUtil.displayBannerAd(self)
...

AdUtil #

...
static func displayBannerAd(_ sender: UIViewController) {
    if (!AppSetting.shared.isRemovedAd) {
        GoogleAdHelper.shared.setBannerUID(Constant.ADMOB.BANNER_UID)
        GoogleAdHelper.shared.setBannerPosition(AdPosition.bottom)
        GoogleAdHelper.shared.displayBannerAd(sender)
    }
}
...

GoogleAdHelper #

...
func displayBannerAd(_ sender: UIViewController) {
    LogUtil.debug("PBExtensionGoogleAd BannerUID \(bannerUID)")
    bannerView = GADBannerView(adSize: GADAdSizeBanner)
    bannerView.adUnitID = bannerUID

    bannerView.delegate = self
    bannerView.rootViewController = sender
    bannerView.tag = 99
    bannerView.load(GADRequest())
}
...

Constants #

struct ADMOB {
    static let BANNER_UID = "ca-app-pub-3940256099942544/2934735716"
    static let INTERSTITIAL_UID = "ca-app-pub-3940256099942544/5135589807"
    static let REWARD_UID = "ca-app-pub-3940256099942544/1712485313"
}

内容的提问来源于stack exchange，提问作者victranit