Why is my Apple ID not adding a discover card to my payment option

What error message do you receive when trying to add the account information.

Apple Card as in Apple Card MasterCard, issued and serviced by Goldman Sachs Bank? Discover Card is a credit card. What exactly are you trying to do?

Can you please explain why we cannot use Discover Credit Card as a payment source for Apple ID? I have tried to reset my network settings and that still does not work.