Hi Janice ...
Click Account right side of the App Store window under Quick Links and sign in.
In the Accout Information window click Edit to the right of Payment Type.
Make sure your credit or debit card account # is correct, along with the expiration date and security code then click Done.
And just so you are aware, your credit or debit card credentials must be associated with the same country where you reside.
Might be help here also > iTunes Store: My credit card's security code or zip code does not match my bank's records