Z0t CohibaRobusto Can you please check your Play Integrity verdict and confirm you have at least the basic level (MEETS_BASIC_INTEGRITY)? You can enable developer mode in the Play Store to expose a hidden menu to check it there without needing a separate Play Integrity checker. This is not the same as developer mode in the operating system, and does not require you to authenticate with your PIN/password.
Can you log into your Google account and make the purchase from the web browser?
Z0t I found the device is not certified play protect
Google intentionally doesn't want to certify GrapheneOS despite being compatible with Android even better than some other phone manufacturers that add aggressive battery optimizations and stuff.