Why does it say my credit card does not match the bank info, when it is a credit card

So today, I tried to purchase an in-app purchase, and almost every time I try to download something with my credit card it says my info is wrong. It says I was missing my security code, and when I typed it in it said it that the info did not match the bank info, and it is a credit card. After that I tried just downloading a free app and now it won't allow me to do that without filling out the rest of the stuff, which I already did, or fixing the problem, which I don't see one. How do I fix this. I tried re-typing EVERYTHING.