I’m trying to add cash to Apple Cash from my Apple card

You can only add money to Apple Cash from a debit card or some prepaid cards. See the following from Add money to Apple Cash - Apple Support

After you set up Apple Cash1, you can add money to your balance using the debit or prepaid cards2 that you already use with Apple Pay in Wallet. If you don’t already have one of these cards in Wallet, you need to add one before you can add money.

Also, the Apple Card is a Mastercard credit card and cannot be used for cash.