Checking to apple card

AppleCard is a credit card; you can’t add cash to it.

I suspect that you mean AppleCash, if so adding money to your AppleCash from checking requires a debit card in your Wallet linked to your checking account.

See more detail here:

https://support.apple.com/en-us/108370

If on the other hand you’re asking how to pay your AppleCard balance, see:

https://support.apple.com/en-us/102534