RE:" I cannot purchase additional iCloud storage ..Can someone advise how I can complete the payment please",
They are many possible reasons for the issue you face. We can not be sure from the info. supplied:
You may find an answer from browsing through the suggestions below:
✽ Your iPad model and iPadOS version may not be up-to-date with your card=provider's security requirements
✽ You could check for any available iPadOS updates in Settings > General > Software updates
You may need to check with your card-provider why payments aren't verified / authenticated : eg:
✽ perhaps your account needs authorisation for intenational payments.
or
✽ if you have disputed a charge from Apple, you may need to authourise payments to Apple again.
Else:
✽ You may need to check whether your payment method is one that apple can accept in your country or region:
Payment methods that you can use with your Apple Account - Apple Support
✽ If needed: How to: Add a payment method to your Apple Account - Apple Support
See what to do: If you need to change or update your Apple Account payment method - Apple Support
How to: Remove a payment method from your Apple Account - Apple Support
✽ You may need to allow 24 hours for account updates to come into effect.
✽ More info: About billing and purchases in Europe - Apple Support