Why can’t I update my iCloud storage?

You don't actually say what the problem is. Apparently something related to your payment? An exact quote of the error would help.

Click on this link to discover how to "Upgrade to iCloud+" --> https://support.apple.com/HT201318

Review this document about what to do "If your payment method is declined in the App Store or iTunes Store" Click here --> https://support.apple.com/HT203005 It mostly just suggests you make sure your payment method is up to date or to try a different one. Apple is probably just checking with the system linking to your financial institution and asking if the payment method is valid. The system is providing a negative response. Make sure the name and address are entered exactly as it appears on your financial institution bills and that the 3 digit CVV is correct. Check with your financial institution about the status of your card.

Make sure this is one of the 'Payment methods that you can use with your Apple ID'; click here --> https://support.apple.com/HT202631 A solution may be to use a method from the same country as the store you are using, but Apple no longer states either way if this is a requirement (it used to say, "Make sure that the billing address for your payment method matches the billing address for your Apple ID" which was more helpful than saying nothing the way it currently does). Also read any footnotes in the document for a payment method.

If you still have problems, contact official Apple Support. Click here --> https://support.apple.com/choose-country-region/ Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service --> "See a list of Apple phone numbers around the world." - https://support.apple.com/HT201232