Hi,
Thank you for getting back to us.
It is always recommended to use the latest versions of the SDK. As of today, the latest version of the Google Mobile Ads Android is 23.6.0.
I have executed our Google sample project and Unity Sample project with the provided information and am unable to load the ads on my end. However, the execution has resulted in the display of an error message “no fill”. The Error code 3, no fill error means that the ad request was successful, but no ad was returned due to lack of ad inventory. You may check this AdMob Error Codes & Logs for more information on different kinds of errors.
The best way to get more information about your issue is the AdMob Help Center. You can also utilize the AdMob Communities for additional support.