I can't subscribe to iCloud+ with an iTunes Gift Card

If you have an iCloud+ plan subscription, you will receive a monthly bill from Apple, check your email box.

You pay monthly for iCloud+ plan storage.

The subscription runs as long as you don't downgrade.

iCloud+ plans and pricing -> iCloud+ plans and pricing - Apple Support

• Accepted payment methods for iCloud+ upgrades include credit cards, debit cards, and your Apple Account balance. If you don't have enough available funds in your Apple Account balance to complete your upgrade, you'll be charged the remaining amount. Apple Store gift cards aren't accepted as payment for upgrading iCloud+. 

You have set your Apple Account balance as your default payment 👍

Please check your mail box, check Apple invoice for your iCloud+ plan.

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Check / update your billing address:

(macOS) Set up your Apple ID payment method and shipping address on Mac

Set up your Apple ID payment method and shipping address on Mac – Apple Support (UK)

or 

Sign in to your Apple ID account page and edit / update your Apple ID details -> https://appleid.apple.com

• edit your billing addres.

or 

To change the billing address you can do (macOS & Windows OS) via iTunes

Start Music (-> Click on iTunes) -> side bar click iTunes Store -> go to Account (top bar) -> View Account -> Sign in -> Apple ID Summary -> Billing Address -> Edit.

or 

(iPhone & iPad) Go to -> Settings -> tap your Profile -> Payment and shipping -> Change Address.

Edit the Address -> tap Done.