Wordpress multisite stack-->How to use a2enmod to enable an Apache2 module

Hello,

I wish to enable the Apache2 module named 'mod_expires' in my Wordpress MultiSite installation. To do this, normally the command (in Ubuntu) is

    sudo a2enmod

Now, as per Ubuntu Docs--

 a2enmod  is  a  script  that  enables  the  specified module within the
   apache2 configuration.   It  does  this  by  creating  symlinks  within
   /etc/apache2/mods-enabled.   Likewise,  a2dismod  disables  a module by
   removing those symlinks.  It is not an error to enable a  module  which
   is already enabled, or to disable one which is already disabled.

EXAMPLES a2enmod imagemap a2dismod mime_magic

   Enables  the  mod_imagemap  module,  and  disables  the  mod_mime_magic
   module.

However, I am getting a "a2enmod:Command not found" error

I checked and in my Wordpress multisite installation, this is the location for the mod_expires module-- "/opt/bitnami/apache2/modules/mod_expires.so"

How do I then enable mod_expires?

Thanks, Arvind.