your purchase could not be completed

If you aren't getting the 'none' option : If you can't remove your last payment method or use no payment method with your Apple ID - Apple Support

If you are getting a message to contact iTunes Support you should do so :

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

- click on your country's flag (the country where you live, 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'