Unable to add or edit payment information in Apple Account

See this article for information about updating payment methods on various platforms (iPhone, Mac, etc.): If you need to change or update your Apple Account payment method - Apple Support

Here is how you update your payment method on an iPhone or iPad:

1. Open the Settings app.

2. Tap your name.

3. Tap Payment & Shipping (formerly Media & Purchases). You might be asked to sign in with your Apple ID.

4. Tap Add Payment Method, enter the new payment method, then tap Done.

5. Tap Edit.

6. Tap the red delete button next to the old payment method, then tap Delete.

Also note:

1) ”If you change your address, the credit or debit card information that you use with your Apple Account might be automatically updated by your financial institution, and you don't need to take action.” (More about that here Protecting against fraudulent Apple.com c… - Apple Community )

2) “If there's an issue [such as you having a subscription or being a Family Sharing Organizer], add a new card first, then remove the old card."

If an app or feature on your device is not working, not appearing, or is restricted (grayed out), it may have been disabled using Screen Time, or by an IT administrator if you are using a device owned by an employer or educational institution.

- Read this support article If your Apple Account/ID or other options are greyed out in Settings —> If you can't sign out due to restrictions, or your Apple Account is grayed out in Settings - Apple Support

Tap Settings > Screen Time > Content & Privacy > scroll to Allow Changes and turn on.

- If the device is managed (Settings > General > VPN & Device Management), contact the I.T. department that is doing the managing.

If you are a member of Family Sharing but have a child’s account, read these about why you cannot add a payment method:

Share apps and purchases with Family Sharing on your iPhone or iPad - Apple Support and How apps, content, and subscriptions from Apple are billed - Apple Support

In this case the Family Sharing Organizer needs to check their payment method is working.