Ad banner on Android doesn't show

Question

I'm adding a banner on my Android app but it doesn't work and I don't know why.

Can you help me please?

2021-09-11 18:11:41.438 6954-7099/com.cristina.xxx E/GmsClient: unable to connect to service: com.google.android.gms.leibniz.events.service.START on com.google.android.gms
2021-09-11 18:11:42.518 6954-7186/com.cristina.xxx E/FA: GoogleService failed to initialize, status: 10, Missing google app id value from from string resources with name google_app_id.
2021-09-11 18:11:42.526 6954-7186/com.cristina.xxx E/FA: Missing google_app_id. Firebase Analytics disabled.
2021-09-11 18:11:42.534 6954-7186/com.cristina.xxx E/FA: Uploading is not possible. App measurement disabled
2021-09-11 18:11:42.692 6954-7007/com.cristina.xxx E/ion: ioctl c0044901 failed with code -1: Invalid argument

MainActivity

protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        getSupportActionBar().hide();
        setContentView(R.layout.activity_main);

        MobileAds.initialize(this, new OnInitializationCompleteListener() {
            @Override
            public void onInitializationComplete(InitializationStatus initializationStatus) {
            }
        });

        mAdView = findViewById(R.id.adView);
        AdRequest adRequest = new AdRequest.Builder().build();
        mAdView.loadAd(adRequest);     
}

Manifest

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

Answer 1

The AdView's size has to be exactly 320x50 dip. It does not show up if it does not get enough room.

Answer 2

I'm answering my self because... In the end was a wrong gradle implementation... Thats the solution:

Manifest file, between application tags (replace yyy with your app id)

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

MainActivity:

MobileAds.initialize(this, new OnInitializationCompleteListener() {
            @Override
            public void onInitializationComplete(InitializationStatus initializationStatus) {
}

activity_main.xml (with banner test ID)

<com.google.android.gms.ads.AdView
            xmlns:ads="http://schemas.android.com/apk/res-auto"
            android:id="@+id/adView"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_centerHorizontal="true"
            ads:adSize="BANNER"
            ads:adUnitId="ca-app-pub-3940256099942544/6300978111">
</com.google.android.gms.ads.AdView>

and finally on gradle:

implementation 'com.google.android.gms:play-services-ads:20.3.0'