I am unable to add payment method on my Apple ID

Why not. It helps a LOT to have that information or else we are guessing. You won't like some of my guesses. ;-)

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 (or Payment & Shipping). You might be asked to sign in with your Apple ID.

4. Tap Manage Payments. Depending upon the iOS version you use 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), 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.

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