Your Purchase Could Not Be Completed For assistance, contact iTunes Support at www.apple.com/support/itunes/ww/.

You should contact the official Apple Support about that and ask them to look into this issue. Only they can access your personal data and settings to find the reason for that.

Choose the phone number form this Apple page:

Contact Apple for support and service - Apple Support

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.