"No ad config" error onAdFailedToLoad even with test ad unit

Android (Java) way:

Minimal reproducible code:

public class MainActivity extends Activity {
    private static final String TAG = "MyTag";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        MobileAds.initialize(this);
        AdRequest adRequest = new AdRequest.Builder().build();
        InterstitialAd.load(this, "ca-app-pub-3940256099942544/1033173712", adRequest,
                new InterstitialAdLoadCallback() {
                    @Override
                    public void onAdLoaded(@NonNull InterstitialAd interstitialAd) {
                        Log.i(TAG, "onAdLoaded");
                        interstitialAd.show(MainActivity.this);
                    }

                    @Override
                    public void onAdFailedToLoad(@NonNull LoadAdError loadAdError) {
                        Log.i(TAG, loadAdError.getMessage());
                    }
                });
    }
}

My build.gradle(app)

implementation 'com.google.android.gms:play-services-ads:20.2.0'

My AndroidManifest.xml:

<meta-data
    android:name="com.google.android.gms.ads.APPLICATION_ID"
    android:value="ca-app-pub-3940256099942544~3347511713"/>

Problem (on Android):

Even using the test Ad ID and test App ID, the ads are not loading up.

---

Flutter (Dart) way:

Minimal reproducible code:

void main() {
  WidgetsFlutterBinding.ensureInitialized();
  MobileAds.instance.initialize();
  runApp(MaterialApp(home: MyApp()));
}

class MyApp extends StatefulWidget {
  @override
  MyAppState createState() => MyAppState();
}

class MyAppState extends State<MyApp> {
  @override
  void initState() {
    super.initState();
    InterstitialAd.load(
      adUnitId: 'ca-app-pub-3940256099942544/1033173712',
      request: AdRequest(),
      adLoadCallback: InterstitialAdLoadCallback(
        onAdLoaded: (ad) => ad.show(),
        onAdFailedToLoad: (e) => print('Failed: $e'),
      ),
    );
  }

  @override
  Widget build(BuildContext context) => Container();
}

Problem (on Flutter):

The test ads show up but when I try to use my own Ad ID and App ID (even in the release mode), it fails with this error.

No ad config.

It's been a week now since I created that ad unit on the Admob. I also have a different app on the Play Store (which is showing ad) but even if I use that app's ID in my code, it fails to load ad with error code 3.

PS: Tried this solution already but it didn't work.

Either of Android or Flutter solution would work for me

from "No ad config" error onAdFailedToLoad even with test ad unit