Hey Frustr8dBrit,
Thanks for using Apple Support Communities! I see from your post you're attempting to update your payment details but getting an error when trying to do so. I definitely want to make sure you're able to keep those details updated, and I'm happy to help.
I first want to share the resource below with you. Sounds like you're already taking the right steps to get that information updated, but this just ensures nothing is skipped. You can also see how to update it on multiple devices if you have another device available to try on.
Change, add, or remove Apple ID payment methods
If possible, I also recommend trying on different Wi-Fi. If you're using an iOS device, ensuring that Low Power Mode isn't enabled would be a good step too. You can check that in Settings > Battery.
Thanks again, and have a great day!