Unable to upgrade my iCloud storage through Apple ID funds

You can use your Apple ID balance as payment method, see:

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)

How to redeem your Apple Gift Card or App Store & iTunes gift card

How to redeem your Apple Gift Card or App Store & iTunes gift card – Apple Support (UK)

Why?

Is the amount needed to upgrade iCloud Storage being shown under the Apple ID balance?

What error message do you get when you upgrade?