App Store Credit Card Verification

I've had an Apple ID for years now, with my associated credit card account, which I use to purchase apps etc. from my iPhone. I've never used the App Store on my MacBook Pro though.

I'm required to update my OS in order to run some new software, but upon opening the App Store on my MacBook, I'm being prompted to enter my Apple ID information, along with my credit card information. However, when I enter my credit card information (which is exactly the same as what I use through my iPhone), it won't accept it, instead displaying a red box vaguely referring me to the Apple Support website.

Has anyone else encountered this problem? If so, how did you fix it? I've double and triple checked that I'm not entering my information wrong, and that it's not a problem with my credit card - neither of these are the issue.

Please help, without being able to log into the App Store, I'm stuck with my old OS, and can't run the software that I desperately need...

Thanks!