Please contact iTunes support to complete this transaction.

See If your payment method is declined in the App Store or iTunes Store https://support.apple.com/en-us/HT203005.

Also see my search link: if your payment method is declined error fix apple.

If you need additional help, use this support link from the document https://support.apple.com/contact or use:

Apple telephone support  for help in solving your problem using this link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362  Be patient sometimes Apple support is closed to immediate phone support.

Apple Support Twitter https://twitter.com/AppleSupport

iTunes worldwide support via text or chat using this link https://support.apple.com/choose-country-region/itunes After choosing your country, see the heading Tell us how we can help.

contact iTunes Support. See the Apple Support articles below for contact information.   

iTunes Support – world wide 

Apple Support schedule phone call       

If necessary, try Twitter

https://twitter.com/AppleSupport.          People have posted it may take an hour or more to get a response.

https://support.apple.com/en-us/HT208856