To learn 'What you can buy with your Apple ID [Apple Account] balance', click here --> https://support.apple.com/HT211187
"Where the Apple Gift Card is available, you can use your Apple ID balance to buy products and accessories at apple.com or in the Apple Store app."
For a list of countries where the Apple Gift Card is available, Click here --> https://support.apple.com/HT202631 and search for 'Apple Gift Card'
(Nov 25, 2021: the USA, Canada, Australia, and Japan)
https://www.apple.com/shop/gift-cards --> "Apple Gift Cards are solely for the purchase of goods and services from the Apple Store, the Apple Store app, apple.com, the App Store, iTunes, Apple Music, Apple TV, Apple Books, and other Apple properties." This list does not include retail Apple Stores.
Terms and Conditions for Apple Gift Cards and Codes, Apple Account Associated Balance, and Content Codes
https://www.apple.com/legal/internet-services/itunes/giftcards/
- "However, only physical and digital gift cards and codes can be used towards purchases in Apple Retail Stores. Your Associated Balance cannot be used for purchases in Apple Retail Stores."
In a nutshell you can only pay for items with account balance if you live in one of the countries listed above and are not doing so in a physical retail store.