Q: App store purchase cannot be completed but we have lots of store credit.

Have you done as the message says and contacted iTunes Support ? If not you will need to do so (these are user-to-user forums) :

- go to http://www.apple.com/support/itunes/ww

- click on your country's flag (the country where you are, and which is therefore the country on your account)

- click on Contact Support towards the top right

- click on Get Help under 'Talk To Us'

Or use this form : https://www.apple.com/emea/support/itunes/contact.html

Have been directed to ITunes Support for assistance.

Then you need to contact Apple directly >  Contact - Official Apple Support