Thanks for the question, and welcome to Apple Support Communities.
I understand you are having issues adding payment information to your account. In my experience, this usually occurs due to two common reasons (though there may be more):
If your account information does not match your credit card, you may get a message stating "The credit card was declined."
To expand on the above, your billing name or address may not match the address listed on your credit card statement. Any discrepancy in punctuation, abbreviation, or capitalization can cause errors, which might prevent you from purchasing from the iTunes Store.
The iTunes Store will place a minimal authorization hold on your credit card each time that you update your account information (equal to around 1 USD). The authorization hold is not a charge. The authorization hold is placed on your credit card to make sure that the information provided matches the information on file with your financial institution. The authorization hold will not be displayed in your iTunes Store purchase history.
If there are insufficient funds in the account to cover the authorization hold, you may experience issues with adding payment information.
For more information, see the following helpful resources: