To setup your custom error page, you will need to use a .htaccess file.
Put the following line(s) in this text file:
ErrorDocument 404 /path_to/not_found_error_page.html
ErrorDocument 403 /path_to/forbidden_error_page.html
ErrorDocument 401 /path_to/authorization_error_page.html
ErrorDocument 500 /path_to/internal_server_error_page.html
Then upload this file to your mainwebsite_html directory or if accesible via File Manager in the folder /var/www/html
Please, make sure to replace the /path_to/.... with the path to your custom HTML page.