There can be multiple causes and solutions for this issue. In no particular order I’ve listed some of the common solutions,
1) Log out of your Apple ID and then log back into your account
2) Turn off you VPN
3) Try using a different connection, if on Wi-Fi use cellular network or vice versa
4) Change your Region on your iPhone to another region then change the region back to US.
Change your Apple ID country or region - Apple Support
5) Update your birthdate on Apple ID
Update the birth date associated with your Apple ID - Apple Support
6) Do a forced restart of your iPhone
Force restart iPhone - Apple Support
7) Update your iOS on iPhone to latest version
Update iOS on iPhone - Apple Support
8) Reset you network settings on your iPhone
Reset your Network Settings. If you're using iOS or iPadOS 15 or later, tap Settings > General > Transfer or Reset [Device] > Reset > Reset Network Settings. If you're using iOS or iPadOS 14 or earlier, tap Settings > General > Reset > Reset Network Settings.
Force restarting you iPhone after completing many of the steps above may help.
If you’re still have issues adding your card after the above, please post a reply.