I am unable to change payment method

Hello Parth_123_,

Thank you for reaching out to Apple Support Communities.

These steps should be helpful:

"How to complete your purchase if a payment failed or was declined

1. On your iPhone or iPad, open the Settings app.
2. Tap your name.
3. Tap Payment & Shipping.
4. Add a different payment method and remove the old one. 
5. Try to complete your purchase."

Additional details can be found here: If your payment method is declined in the App Store or iTunes Store

If the issue persists, are you able to test with another payment method?

Cheers!

Update your payment method on your iPhone or iPad

1. Open the Settings app.
2. Tap your name.
3. Tap Payment & Delivery. You may 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.