Apple Account failing to connect my valid card for app downloads

You need to contact your card issuer.

One other possibility: Do you have a 3rd party VPN installed, even if you are not using it? If so, delete the VPN profile in Settings/General/VPN & Device Management/VPN, then restart your device. Don't just turn it off; delete the profile. Updates sometimes need to have VPN to be removed and reinstalled. If that fixes it, you can try reinstalling VPN, but probably shouldn't.

Can you provide the exact wording of the error message please? I have never see anybody mention the one you post. If that is indeed to wording of the error message then I guess it is possible that Apple truly could not contact the issuer of the card to verify its authenticity. You should contact the issuer about this. Their system may have been down or something. Have you tried again? Does this persist over several days?

Here are a variety of resources helping with connectivity issues:

Restart your equipment and/or sign out and back in again, and try again.

If you can't connect to the App Store, iTunes Store, or other Apple services, read this support article --> If you can't connect to the App Store, iTunes Store, or other Apple services - Apple Support

Here's a related article about what to do if you can't connect or sign in to iCloud --> If you can't connect or sign in to iCloud - Apple Support

If your iPhone or iPad won’t connect to a Wi-Fi network, or if you have related connectivity issues, read this support article --> If you can't connect to Wi-Fi on your iPhone or iPad - Apple Support

Here's a case where using a VPN prevented sign-in --> AppStore - Apple Community

Remove the VPN entirely, don't just turn off.

If none of the resources provide a solution, contact Apple for assistance:

For support, click here --> Choose your country or region - Official Apple Support Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service --> "See a list of Apple phone numbers around the world."

Click here --> Contact Apple Support - Apple Support