The error itself indicates that the page does not exist. At the time of editing, it is not saved, but is in the cache and will be available only after saving your editing. But more information is needed. What site? What platform? Template or created site? If the site is created, then refer to your web development services company . Let them check the code for consistency. In the template, by the way, this page can be simply missing (I'm about 404). Therefore, such a mistake. It is necessary to look at a concrete example.