When you create a web in FrontPage, or install FrontPage extensions on
a web site, you always get folders named _private and images within
the web.
If you do not have a _private folder, then either you do not have the
FrontPage extensions installed and the Guestbook will never work,
or the web is not open in FrontPage and the Guestbook will not work,
or you have deleted the _private folder.
The solution (if a server-based web) is to run Server-Health on the
web, or to re-install the extensions. This should recreate the
_private folder with the correct permissions. You may need to talk to
your host.
If disc-based web, try creating the _private folder in your web, but
it will probably be better to open a new web, and Publish your
existing files into it.