Why can’t I purchase with my Apple balance .

Maybe it is something for which balance cannot be used? Including any error messages would help us determine what is the exact issue. This document outlines what you can buy with your Apple ID [Apple Account] balance --> What you can buy with your Apple Gift Card or Apple Account balance - Apple Support

Note that you do not need to do anything with your payment method order. Account balance is always charged first unless it is something for which account balance cannot be used as payment. You couldn't undo that even if you wanted to and there is certainly no need to try to change anything.

From this support article --> How apps, content, and subscriptions from Apple are billed - Apple Support

"Which payment method is charged?

When you buy items or pay for subscriptions, Apple automatically attempts to charge your payment methods in this order:

1. Apple attempts to use your Apple ID balance to cover the full amount.

2. If you don't have Apple ID balance, you don't have enough to cover the full amount, or an item cannot be paid for with Apple ID balance, Apple attempts to charge your primary payment method (typically a credit or debit card, but varies according to country). The primary payment method is at the top of the list in Settings on your iPhone. You can control which payment method is at the top of the list. [See: Add a payment method to your Apple ID - Apple Support ]

3. If there's a problem with the primary payment method, or if the primary payment method can't be charged for the full remaining amount, Apple attempts to charge your other payment methods in order from top to bottom. If none of them can be charged, Apple may try again."

See -> Add a payment method to your Apple ID

Add a payment method to your Apple ID – Apple Support (UK)

• if you want to use your Apple Account balance, you need to set your Apple Account balance as your default payment.