Purchase could not be completed

You may need to have a valid payment method on file, even if the app you want to download is free.

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

See also ⤵️

Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

FYI: Not all credit & debit cards seem to work with App Store / iTunes Store, try a different one, or preferably a credit card. If your card is not accepted as a paymenf method, you need to contact your bank.

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.

Get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK) 

See the Apple Recommended answer to the same question here.

purchase could not be completed purchased… - Apple Community