Hi
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.)
This message indicates that the error is coming from your web server, not from Joomla. This would indicate that you did not added a valid .htaccess file on your server.
A .htaccess file is required by Joomla if you want to use URL rewriting. Joomla has a very good documentation on this here.
Usually, just renaming the htaccess.txt file in your Joomla installation to .htaccess will be enough to solve this as the standard Joomla file is ok for most types of servers.
Best regards
Yannick Gaultier
weeblr.com / @weeblr