Did you remember to add the correct filename extension to the end of the page you wanted to visit?
Remember that on this site, even webpages need the extension ".html" at the end of their addresses.
...Oh, and since you've been poking around a bit, I'll let you in on a little secret:
Some of the pages are *hidden.*
This means that you can't reach them by following links from the main site.
The only way to find them is to enter a name in the address bar and hope you're right.
And even the links that are available from the main site may not be immediately obvious...
In any case, if you decide to go exploring, you'll be seeing this screen a lot,
so take it in stride!