Hello icrackofdawn,
Thank you for using Apple Support Communities. Sounds like you're unable to update your payment method. Mainly because the option to do so is blank and doesn't let you input anything. If so, weblike to help out and recommend a couple of steps to troubleshoot this issue. Follow the steps below:
1.) If you can't update your payment method, make sure your region supports the payment type: Payment methods that you can use with your Apple ID - Apple Support
2.) If everything checks out, then try checking to see if you have any restrictions enabled on your iPhone. Sometimes this can prevent you from editing certain settings on your device. Go to Settings > Screen Time > Content & Privacy Restrictions to turn it off.
3.) Try refreshing things by simply restarting your iPhone. Restart your iPhone - Apple Support
We hope these steps help you out. Thank you for using Apple Support Communities. Take care.