Can be seen while browsing the page (when the page is not available) or in the browser developer tool (displays missing files).
The 404 or Not Found error message is a HTTP standard response code indicating that the client was able to communicate with the server, but the server could not find what was requested.
For missing files make sure the missing files exist in the specified directories. Try to change the file permissions to 755 or 777.
For missing pages please make sure the page URL is valid and the page exists.