cannot load custom-header

Newbie here- I’ve only done WP content management before and now trying to create a new site from scratch. I have tried several different themes and cannot access the theme options or header in any of them. I get error message Cannot load custom-header and Cannot load theme.
I have uninstalled and reinstalled the whole WP package through FileZilla, but still the same problem. I have no extra plug-ins.
HELP please!