Why I’m unable to add payment to my iCloud

Verify your payment information:

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

https://appleid.apple.com

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

See also the following Apple articles:

Payment methods you can use with your Apple ID

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

Change, add or remove Apple ID payment methods

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

• Add first the new card, then remove the old one.

Add money to your Apple ID balance

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

If you can't remove a payment method from your Apple ID

If you can't remove a payment method from your Apple ID - Apple Support

If you update your payment method or add funds to your Apple ID balance, It would take about 1 to 3 business days in order to process.

If you're still unable to update your payment method, Contact Apple Support.