can't buy in app purchases on iPad V3

Hi I've been trying to purchase an in app extra but every time I try to I get a message saying Your Purchase Could Not Be Completed for assisance please contact iTunes support at www.apple.com/support/itunes/ww/. I can buy apps and when I view my apple I.D in the payment scrren is says Click and buy Is Not Supported on this device go toiTunes on your computer I've tried to sign in and out reset the iPad I can't see what the problem is.