These are user-to-user forums, we won't know why you are getting the message, but it could be due to a number of reasons e.g. too many incorrect passwords, a problem with a previous purchase, if you are trying to buy them when not in your own country, ...
If you haven't already done so then you can contact iTunes support via this page and ask them why it's appearing : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page