I got a message:
Add or update app-ads.txt
Your app-ads.txt file is either missing or not valid. To prevent a significant loss in ad revenue, add or update your app-ads.txt.
Publish your app-ads.txt on the root of your developer website (for example, sampledomain.com/app-ads.txt). Make sure that the domain is entered exactly as listed on Google Play or the App Store.
I don't have a developer website.
My Requirement: How to setup this app-ads.txt file for free? (I've no domain or website)
Create a Blog in https://www.blogger.com/
click on settings and then in Monetization switch on Enable custom ads.txt
In Errors and redirects click on Custom redirects and add app-ads.txt and then click save