This document explains how to add a payment method to your Apple ID on a Mac, Apple mobile devices, PC, or online --> Add a payment method to your Apple ID - Apple Support
On an iPhone or iPad (note that the details may vary slightly depending upon which iOS version you are running) :
1. Open the Settings app.
2. Tap your name.
3. Tap Media& Purchases (formerly Payment & Shipping). You might be asked to sign in with your Apple ID.
4. Tap Manage Payments. If using an older iOS version you might not see this step.
5. Tap Add Payment Method. Enter the new payment method, then tap Done.
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 and turn on.
- If the device is managed (Settings > General > VPN & Device Management), contact the IT department that is doing the managing.