By 'can't do in-app purchases' you mean ... ? e.g. does the 'buy' button not work, do you get any error messages ... ?
If you are getting a message to contact iTunes Support then you can do so via this link and ask them why the message is appearing (we are fellow users here on these forums, we won't know why) : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption
If it's a different problem ... ?