Invalid Credit Card Security Code

I have recently noticed that I can no longer make purchases on the iTunes platform. I get a message from iTunes/Apple instructing me to check my card details. So I go over to my settings and change my credit card details. After inputting all the necessary details, I can't save because I get the message, "Payment Information Could Not Be Updated. Invalid Credit Card Security Code." I'm very sure the problem is not on my side because I have tried using more than one credit card, but I keep getting the same message.