This happened due to updated AdMob SDK. Now you need to add appID in your manifest file.
<application> . . .
<meta-data
android:name="com.google.android.gms.ads.APPLICATION_ID"
android:value="ca-app-pub-################~##########"/>
</application>
You can get appID from here –>Google addmob –>Select app –> App Settings –> App ID.
You can read more from here ads-developers.
If you want to use test ads before putting your own id check the AdMob test ids from here developers.google.
Hope this will help you. Thanks 🙂