I also got this error when I try to purchase an app from iTunes Store on my iPad.
"Your Purchase Could Not Be Completed - For assistance, contact iTunes Support at www.apple.com/support/itunes/ww/."
When I go to this URL, I get the page to select the Country. Which really no help at all.
What I did then was try to make the same purchase on my iPhone. On this occasion iTunes Store advised me I had to verify a new device. In doing so I discovered I needed to update my CC details. Once I did this my iPad stopped getting the error.
Seems the iPad on this version does not do the Account Verification step when it is required. Instead you get this error.