All pages and databases must be subpages
If you're seeing an error message “Page not found” the issue is likely that you're linking to a page that's outside of your main Notion page you initially set up with Super.
We can only read the subpages underneath that main Notion homepage you have linked to Super, so linking to anything outside of it will result in seeing an error message. Pages outside of the main Notion page will not be visible in Super for security reasons to prevent accidental publishing of your Notion pages in Super, even if those pages are Public pages.
It should start working once you move the page that's giving you an error within your main page inside Notion.
Make sure the Notion pages are shared publicly
Sometime the page sharing permissions of Notion pages can be unset or changed back to private. This will also cause a Super page to “404”.
Make sure the Notion pages for your site are set to Public by going to the Share menu of the Notion page.