I am not able to purchase iCloud storage and this is the message which shows after I try to make payment

- Are you [still] using a valid payment method? --> Payment methods that you can use in the iTunes Store, App Store, and iBooks Store - (Prepaid cards may not work or may only work erratically. Cards that previously worked may not always work.) - https://support.apple.com/HT202631

- If your payment method is declined in the App Store or iTunes Store - various reasons why payment method is declined. - https://support.apple.com/HT203005

- If this abruptly started happening try removing and re-entering your payment information - Change or remove your Apple ID payment information - https://support.apple.com/HT201266

- Do the name, address (all spelling, letter case and punctuation), and card numbers exactly match those you have entered? You didn't mis-type anything?

- You entered the expiration date and 3 digit security code for the card? Correctly?

- The country for the bank and your address are the same as the iTunes Store country you are using?

- See the end of the declined payment document linked above for special measures that may be necessary for some cards, plus about contacting your financial institution to make sure your records match.

If you still have a problem contact Apple:

iTunes Store - Contact Us - https://www.apple.com/emea/support/itunes/contact.html

or

Worldwide iTunes support - https://support.apple.com/country-selector/itunes