i cant remove old payment method or add new payment method

To find out how to remove a payment method, and what to do if you can't remove your last payment method or use no payment method with your Apple ID, read this article ➔ Remove a payment method from your Apple ID - Apple Support

Unexpired subscriptions and Family Sharing organizers must always have a payment method present. To edit the cards used for those you may need to first add the new card and move it to the top payment method position before removing the old card (see: If you need to change or update your Apple ID payment method - Apple Support ).

If an app or feature on your device is not working, not appearing, or is restricted (grayed out), a frequent reason is 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 ID or other options are greyed out in Settings —> If your Apple ID is greyed out in Settings - Apple Support

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