i have not set my security questions but it still asks me for them when i try to purchase applications

I tryed to download an app which cost $6.99. I had $10.00 on my account. when I clicked buy it asked me for my security questions, which I have not set. I tryed to go on applid.apple.com and cange them but they asked for them again. I really need help so if anyone could be knd and help me that would be appriciated.