Welcome to Justanswer.com, my name is Dustin.I'm sorry to hear about the issue you are having, I will do my best to help you resolve it.
Depending on what company or program you are trying to make purchases through, itunes may be blocking them or blocking your card, or your card could be blocking itunes due to the frequency in the last few years of scams through itunes.
So let me ask you this. DO NOT mention any of your card numbers or details, but what company is your bank or credit card with?
Also, what company or program is it you are trying to make purchases from through your Ipad?