Where can I mail a check to pay off Apple Card?

No. There is no mailing Address. You cannot pay the card with a check. Its designed to be paid directly on your iPhone.

You pay directly on your iPhone, using your Bank account which only you have access to.

Nobody but you has access to your account. Neither Apple nor Goldman Sachs have access to your bank account.

Normally you pay from the phone using the following instructions: How to make Apple Card payments - Apple Support

Also see the following from that link:

If your iPhone or iPad is unavailable

If your iPhone is missing or stolen, you can call an Apple Card specialist to make a payment. You can also ask an Apple Card specialist to help you set up scheduled payments so you won't miss a payment.

They have not published a payment address that I know of. Use the instructions from my previous reply to see if they will provide one:

you can call an Apple Card specialist to make a payment.