Customize the 404 page for your site. The 404 page is used in case an invalid or unreachable URL is entered as an internal link of the site domain; that is, in lieu of any page that can not be rendered. If a site visitor tries to access a non-existent page on your site, they will see your customized 404 page as opposed to the generic 404 page that we provide by default.
To customize the 404 page:
NOTE: The 404 page does not display under the Pages menu or the Pages drop-down list in the top left of the menu bar of the editor.