Required XML attribute “adSize” was missing

<com.google.android.gms.ads.AdView
    xmlns:ads="http://schemas.android.com/apk/res-auto"
    android:id="@+id/adView"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    ads:adUnitId="YOUR_AD_UNIT_ID"
    ads:adSize="BANNER"/>

basically, add this line in the ads xml

xmlns:ads="http://schemas.android.com/apk/res-auto"