My card won't let me update apps

If you are getting a message saying that it's declined it's declined - what is or isn't accepted by iTunes can obviously change over time. Either see if you get the 'none' option so that you can remove the invalid card, or replace it with a valid one : Change, add, or remove Apple ID payment methods - Apple Support or https://appleid.apple.com/account/manage

Why won't it, what happens ?

The card may be valid elsewhere, they does not mean that it passes the iTunes validation and is valid for iTunes. Either replace it with a valid card, or see if you can remove it