Hi,
Have you tried to switch user account mentioned in this article to check the issue?
Fix Windows Store error: Your purchase couldn’t be completed
http://www.ghacks.net/2012/06/25/fix-windows-store-error-your-purchase-couldnt-be-completed/
If the issue still persists, please contact live account support to reset your account or change the password for your Microsoft account to see what’s going on.
http://support.microsoft.com/contactus/
Hope these could be helpful.
Kate Li
TechNet Community Support
