unable to add payment method

Sign in to your Apple ID account page and edit your Apple ID details.

https://appleid.apple.com

On the Sidebar click Payment methods -> then tap Payments Methods and tap Manage payment method.

Or -> Change, add or remove Apple ID payment methods

Change, add or remove Apple ID payment methods – Apple Support (UK)

See also -> Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

You can use also your Apple ID balance as payment method:

Add money to your Apple ID balance

Add money to your Apple ID balance – Apple Support (UK)

What you can buy with your Apple Gift Card or Apple ID balance

What you can buy with your Apple Gift Card or Apple ID balance – Apple Support (UK)