Can't remove credit card; gift card for subscriptions

Since you have had subscriptions, you almost certainly need to retain a valid payment method.

Payment methods that you can use with your Apple Account - Apple Support

Without revealing personal information, it’s hard to imagine why you would wish to transition from having a valid payment method on file to having no valid payment method (the Apple Gift Card being irrelevant) when you’re not planning to use the card anyway. What’s the harm in keeping it there?

You are, of course, the only user of this Apple Account, right? You certainly should be.

Make sure that each family member has a unique Apple Account - Apple Support

In that case, the Apple Gift Card would be charged for the subscriptions first, as long as it retains a balance. Keeping at least one payment method on file is likely required.

cc189 wrote:

I just wanted the credit card on my account.

It’s there! Goal achieved.