Why can't I buy donuts for my simpsons tapped out

What happens when you try to buy them, 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 for help (we are fellow users here on these forums, we won't know why the message is appearing) : 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 ... ?