Unable to make purchase even if I have balance in Apple Account

There are a restrictions on what you can use your Apple Account funds for. Here is a short list.

Apple Gift Card and Apple Account balance can't be used for some purchases

• You can't use your Apple Account balance to send gifts or gift cards.
• For some recurring charges, like AppleCare+, you can't start your subscription with Apple Account balance. But as long as you keep another valid payment method on file and your Apple Account balance would cover an entire charge, your recurring charge will pull from your Apple Account balance.*
• Some subscriptions might not be charged to your Apple Account balance.
• If you use Family Sharing, your family members can't spend your Apple Account balance. Funds that you add to your Apple Account balance are available only to you. Learn more about how purchases bill to your Apple Account balance.

* In some countries and regions, your Apple Account balance can’t be used for some subscriptions like AppleCare+. Check if Apple Account balance in your country or region isn’t supported.

I’ve linked an Apple Support article below that lists what you can use your Apple Account funds for.

What you can buy with your Apple Gift Card or Apple Account balance - Apple Support

It would help if you told us any error messages you see. It could be either because you cannot buy the item with account balance (as a previous post outlined), or you are confusing seeing a message asking you to enter a payment method with not being able to use your balance to pay for the item. In the second case, Apple will frequently require the presence of a card of some kind even if an item is free. The card may not be charged but it has to be there if Apple wants to see it.

Ref.:

Add a payment method to your Apple Account - Apple Support

"To make purchases from the App Store and use subscriptions like iCloud+ and Apple Music, you need at least one payment method on file." Note that there may be a differences between “purchasing” an app for money, and simply “getting” one for free. I get free ones with no payment method.

If you can’t download or update apps on your iPhone or iPad - Apple Support

- "You might need to have a payment method on file, even if the app that you want to download is free."

Remove a payment method from your Apple Account - Apple Support

- If you have any active subscriptions, you must have at least one payment method on file other than Apple ID credit.

- You might not be able to remove a payment method if you have a balance due.

Sign in to your Apple Account - Apple Support

- "When you sign in to the App Store and other media services for the first time, you might need to add a credit card or other payment information."

How apps, content, and subscriptions from Apple are billed - Apple Support

- You can’t charge gifts to your Apple ID balance. If you send a gift or gift card, your primary payment method is charged.

- Some subscriptions might not be charged to your Apple ID balance.

What you can buy with your Apple Gift Card or Apple Account balance - Apple Support

"Some purchases might require that you have a credit card on file, even if you use your Apple Account balance to make the purchase."