Apple ID payment methods
How do I get the Edit button on Apple ID payments to be selectable?
If this is on a device, do you have access to another WiFi network where you can try? That has worked for several people.
Try a restart.
Restart your iPhone, iPad, or iPod touch
Try a forced restart.
If necessary, contact iTunes Support. Either use the form or scroll to the bottom of the page in the 2nd link. Follow through the screens to get a chat or telephone option. You may need to pick a problem that is slightly different than yours to get a chat option.