You can use your debit card though.
That's probably because /e/OS uses microG, which is vastly inferior to Sandboxed Google Play on GrapheneOS, and has much worse app compatibility. You should also know that /e/OS is a highly insecure OS, and both Fairphone and Murena are constantly misleading their customers with false marketing and false promises.
It would be amazing if grapheneos would support microG as an option. But they are too much "not invented here" for that to happen.
What is "caveman" about a payment method that doesn't need to have a charged battery?
https://discuss.grapheneos.org/d/24134-devices-lacking-stand...