If you're getting this message on your Apache installation, coupled with a 500 Internal Server Error message, it means you've used the
ExpiresActive command in a .htaccess file or in the apache2.conf (or httpd.conf) file without including the Expires module. Fortunately, it's an easy fix, especially on Ubuntu systems:
Simply run as root:
ln -s /etc/apache2/mods-available/expires.load /etc/apache2/mods-enabled/
On non-Ubuntu/Debian systems, you'll need to edit the apache2.conf (or httpd.conf) file to enable the module. See the Apache Documentation here for more info.
Once that's done, restart your Apache2 server:
service apache2 restart
Your server will now have the expires module installed and the error message should be gone!
If you do not have shell access to the server or your web host controls modules you may need to speak with them to get it enabled. If that fails, you may need to edit the .htaccess file which is using mod_expires and comment it out. You may be able to use Headers instead. It's not as flexible, but can still be made to work, especially if all you need is a far future expires.
<FilesMatch ".(jpg|png|gif|js|css|jpeg|ico)$"> Header set Expires "Mon, 1 Jan 2020 12:00:00 GMT" Header set Cache-Control "max-age=29030400, public" </FilesMatch>