Need help buying storage with apple funds

Please see these Apple Documents.

Buy more iCloud storage https://support.apple.com/en-us/HT201318

iCloud storage plans and pricing https://support.apple.com/en-us/HT201238

Accepted payment methods for iCloud storage upgrades include credit cards, debit cards, and your Apple ID balance.

And see: Change, add, or remove Apple ID payment methods - Apple Support

If you have a subscription for more iCloud storage, you need to have a credit card on file. If you have a balance in your App Store/iTunes account, charges should be deducted from that before your credit card is charged.

If that isn't what is happening, you need to provide more information about the problem you are having and any error messages you are seeing.