Accepted payment methods for iCloud storage upgrades include credit cards, debit cards, and iTunes Store credit. If you don't have enough iTunes store credit to complete your upgrade, you'll be charged the remaining amount. Apple Store gift cards aren't accepted as payment for upgrading iCloud storage.
See if any of the following articles can help:
- Payment methods you can use in the iTunes Store, App Store, and iBooks Store - Apple Support
- Buy more iCloud storage - Apple Support