During my attempt to get the code by redeeming it in the app store, the security code(itune gift code) was requested for again,pls what do I do?

I tried to download a free app in the app store and I was asked to select whether to use an existing id or to create a new one, so I chose to use my existing id. After agreeing to the terms and filling some other things, the only thing left is to put in a security code which is underneath "iTunes gifts and gift card code" and that's what I have no idea about. The code is being requested for when I make attempt to download anything (whether free or to purchase). I also tried to redeem it at the bottom of the app store but again , I still need the code.