My purchase could not be complete

If you get a message -> Your purchase could not be completed, you need to Add a payment method to your Apple ID – Apple Support (UK)

• If you want to change your card, add first the new card, then remove the old one.
• or if you want to use your Apple Account balance, you need to set your Apple Account balance as your default payment.

👉 So you can use your Apple account has 300 Australian dollars,

Add money to your Apple Account balance -> Add money to your Apple Account balance - Apple Support

If you have set your Apple Account balance as your default payment method, your default payment method will be used first by Apple when you need to make a payment.

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

If you update your payment method or add funds to your Apple Account balance, It would take about 1 to 3 business days in order to process.

If you're still unable to update your payment method, Contact Apple Support.

If you still need help, get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK)