Page - 404 Error
Create a new page
Create a new node of type Basic page
Use below sample code to create this page.
We are sorry, but the page you were looking for doesn't exist.
Set this page as default Error 404 page
When creating page, make sure you select Full HTML as Text format.
You have now successfully created Error 404 (Page not found) page.
Navigate to: Administration >> Configuration >> System
And set this page as your Error 404 page and click Save Configuration button.