It is strange you do not have the
expires_module line in the config file
httpd.conf, could you check if it is present with this command:
$ cat /opt/bitnami/apache2/conf/httpd.conf | grep expires_module
That should output the line you need to uncomment.
Regarding the missing command, please try running the command without the initial
$, as this is used to represent in written form that the line is a command to be run on a console. There is no need for the instance to be restarted, running the command to restart apache should suffice for the change to take effect.
Francisco de Paz