Says unable to purchase on this account close or restart app
Do you have Google Play pass? You probably have: make sure Diablo Immortal account is signed in with account that has Google Play Pass, and you accepted that offer in the Play Store (monthly).
I did see a lapse of time between when I accepted offer and it was available to me. But I know my issues was Google Play based and not battle.net based.
Good luck, update if you learn more