Can't update payment method. Add payment button is gray

If you are the Family Sharing Organizer you need to add a valid payment method or get the expired one working again. See this article about updating payment methods on various platforms (iPhone, Mac, etc.): If you need to change or update your Apple ID payment method - Apple Support

If you are not the Organizer, you need to ask the Organizer to correct the issue or else you need to leave the family group so you can add your own payment method.

Are you a member of a family sharing group and share purchases is on? then you cannot add a payment method yourself, you must ask your family organizer to do that.

Is screen time turned on? You might want tot turn off content and privacy restrictions via settings>screen time>content and privacy restrictions and turn those off. See if you can add a payment method after this

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 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.

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

Yeah If you'e not the organizer great chance that they have disabled this feature in the family setup