Sometimes you need to wait
When you edit your site with Stastic, it is built with GitHub pages every time it changes. This takes about 20 to 30 seconds. During this time your changes such as creating a new page are not live, so if you visit this page it will display 404 not found page.
Also note that you may try to clear your browser’s cache, this might help.
Sometimes it is a good thing
If your site has been built and you still have if 404 not found page instead of an existing page, this is probably due to GitHub excellent server side cache. This means that GitHub has a system to optimize the speed at which your website is delivered and because of the system a new page take a while before it is accessible.
Use Stastic preview to bypass GitHub cache and always see the latest version of your website*