I got problem to solve, which when I buy. It could not purchase completed, what is the problem?

If you get a message -> Your purchase could not be completed, you need to update your payment method or add a new one. 

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

If you update your payment method or add funds to your Apple ID 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.

Click here for more support and assistance.

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

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

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