Hello elizabethbabyrn and welcome to Apple Support Communities,
If we understand your post correctly, you're unable to complete the purchase when trying to add money to your account. We’d like to help.
We see you were viewing "Add a payment method to your Apple ID - Apple Support" and it did not resolve your issue.
Follow the steps in this article to try your payment again:
If your payment method is declined in the App Store or iTunes Store - Apple Support
Let us know if that resolves this issue for you.
Thanks for using the Apple Support Communities.