App Store 'your session has timed out'

Whenever I try to buy an app on the app store it says 'your session has timed out'. I've read up on this and most people say that you have to go to settings and enter your credit card payment information. This would work for me, but I don't have a credit card so I can't enter any information there in the first place. I only ever buy things on ITunes of the App Store with gift cards. I don't know how else to fix this problem, please help.