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If you get a message -> Your purchase could not be completed, you need to update your payment method or add a new one.
See also the following Apple articles:
Payment methods you can use with your Apple ID
Payment methods you can use with your Apple ID – Apple Support (UK)
Add money to your Apple ID balance
Add money to your Apple Account balance – 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.