Why can’t I remove my credit card

Is it giving you a warning about subscriptions? If you already selected remove payment method and you still see it as an option when you are adding a new card that is normal and the old card will drop off eventually. There is no time frame for that, it just happens.

Here is a link that should help - Change, add, or remove Apple ID payment methods - Apple Support

and here - If you can't remove your last payment method or use no payment method with your Apple ID - Apple Support

and here - Manage the cards that you use with Apple Pay - Apple Support