When I try to edit theme’s header, message, “sorry, that file cannot be edited.”

When I try to edit theme’s header, I get the error message, “sorry, that file cannot be edited.” Using wp9.2, theme is digg 3-column, but got the same message with f2 theme. I thought it was a problem with the theme (sorry Small Potato), obviously it’s not. The editor DID work before. I don’t know what I did to make it stop working, especially since I get same message in more than one theme.