apple card payment

You can only make Apple Card payments with a Bank Account. It won't let you use another card.

See this article: How to make Apple Card payments - Apple Support

Taken from that article: "Add a bank account that you can use to make payments, or use Apple Cash to help pay off your Apple Card balance. The first time you choose to add a bank account, you're asked if you want to add the same bank account that you use with Apple Cash."