Could not write sitemap to file

The plugin was installed and ran but fails with “Could not write sitemap to file”. This site requires member access. Since you did not log in to the site, I am not sure if you would see all of the links. And why should a regular, no member who has not logged in be able to write the file to the root folder?

Thanks in advance for your help or comment.

The page I need help with: [log in to see the link]