Too many recently failed requests for ad unit ID (AdMob)

Too many recently failed requests for ad unit ID: ca-app-pub-XXXX. You must wait a few seconds before making another ad request.

I erased all contents of simulator and restarted it, but it didn't work at all. I guess it might be happened because of previous failed requests, I have no idea how to solve it. I just waited a minutes, but ad didn't show up. Also I don't understand what you must wait a few seconds before making another ad request means. Has anyone solved this?

import 'package:admob_flutter/admob_flutter.dart';

class _HomeState extends State<Home> {
AdmobBannerSize bannerSize;
AdmobInterstitial interstitialAd;

@override
  void initState() {
    super.initState();
    Admob.initialize(Platform.isIOS
        ? 'ca-app-pub-XXXXXXXXXXXXXXXXXXXXXXXXX'
        : 'ca-app-pub-XXXXXXXXXXXXXXXXXXXXXXXXX');
    interstitialAd = AdmobInterstitial(
      adUnitId: Platform.isIOS
          ? 'ca-app-pub-XXXXXXXXXXXXXXXXXXXXXXXXX'
          : 'ca-app-pub-XXXXXXXXXXXXXXXXXXXXXXXXX',
      listener: (AdmobAdEvent event, Map<String, dynamic> args) {
        print('$event');
        if (event == AdmobAdEvent.closed) {
          interstitialAd.load();
        }
      },
    );
    interstitialAd.load();
  }


 @override
  Widget build(BuildContext context) {
  return Scaffold(
        resizeToAvoidBottomPadding: false,
        resizeToAvoidBottomInset: true,
        body: ListView(children: <Widget>[
           Container(
              width: MediaQuery.of(context).size.width - 50.0,
              height: 50,
              margin: EdgeInsets.only(left: 25, right: 25, top: 5),
              decoration: BoxDecoration(
                  color: Color(0xFF337B6D),
                  borderRadius: BorderRadius.circular(25)),
              child: AdmobBanner(
                adUnitId: Platform.isIOS
                    ? 'ca-app-pub-XXXXXXXXXXXXXXXXXXXXXXXXX'
                    : 'ca-app-pub-XXXXXXXXXXXXXXXXXXXXXXXXX',
                adSize: AdmobBannerSize.BANNER,
                listener: (AdmobAdEvent event, Map<String, dynamic> args) {
                  print('$event');
                },
              )
            )
          ]
        )
}