Need help with ads, Ads not showing even published on Google Play

Hi everyone, i recently made an app and published it on Google Play Store, everything was working well, but when I placed ads in it, it was working companion, but when I published it, and downloaded it from Google play, ads were not showing and following error was getting shown:

Please help me, I need help urgently, because I launched an event in a school in my city, where I’ll explain them my app, and i will get so many installs, and if ads doesn’t works, it will be loss full for me, so please help me

Google Play Link Of The App :

Thank you, waiting for help.

@Kodular #help

Are you using latest SDK?

Yeah I am using Latest SDK