Which Android version are you using and do you have Google Play services installed?
Google Play services, Yes I have installed.
Android version 9 PKQ1.181203.001
(Android 9 Pie)
Issue:

I just realized this application is not available worldwide. It is only for Germany. So I can not download It.
Actually I wanted to scan the barcode given by corona test center to determine the results of covid-19 test.
Hello @rebelgiri,
the Corona-Warn app currently is available in multiple European countries. Please check out the FAQ: https://www.coronawarn.app/en/faq/#international.
You should be able to switch to one of these PlayStores to be able to download the application.
Thanks,
LMM
Corona-Warn-App Open Source Team
Now, the problem is Google Play Store allows to change region only once in a year.
@rebelgiri which country are you currently using?
If its not yet supported, you can create an entry in the wishlist repo for your country or upvote an existing one: https://github.com/corona-warn-app/cwa-backlog/issues/9#issuecomment-648745301
@rebelgiri you can also grab recent APK and sideload it: https://www.apkmirror.com/apk/robert-koch-institut/corona-warn-app/corona-warn-app-1-1-1-release/corona-warn-app-1-1-1-android-apk-download/
Note that it won't update automatically, and as soon as CWA is released in your Play Store country you should uninstall it and install from Play Store, but for now I guess using it like this is better than not using at all.
Thank you all for your support, I will close this issue as problem is resolved. I am using APK for now as suggested by @kbobrowski.
@daimpi Yes I will upvote it.
Most helpful comment
@rebelgiri you can also grab recent APK and sideload it: https://www.apkmirror.com/apk/robert-koch-institut/corona-warn-app/corona-warn-app-1-1-1-release/corona-warn-app-1-1-1-android-apk-download/
Note that it won't update automatically, and as soon as CWA is released in your Play Store country you should uninstall it and install from Play Store, but for now I guess using it like this is better than not using at all.