Are you sure any Antivirus is not active?

@msaroid We will discuss on any such possibility and will implement if possible.

Just because you need to make a purchase for genuine copy with us to be able to build AAB.

Using third party pirated copies will not work.

@impurnendu It varies from device to device. We recommend 720x1280 & 1080*1920 sizes because these fits best on most of the devices.

Seems like some AntiVirus is blocking the installation. You can try it by allowing installer through the antivirus program.

You need to debug your app to check console error and get to know the exact issue:
https://websitetoapk.com/docs/debugging-with-chrome.html

@sefaaydin I would like to inform that this is not causing any issues. The code on our website is entirely correct and tested. However, we are still glad that you resolved it anyway.

Since this is your specific issue, please raise a support ticket:
https://websitetoapk.com/support/submit_ticket

Usually if they don't keep clicking on your ads intentionally, there won't be any issue and no need to list each of them as test device. Further, you can connect to Google AdMob support if you have some exceptional case.

First of all, you should have Material+ version, after that, follow below tutorial:
https://websitetoapk.com/docs/setting-up-firebase-fcm.html