Apache 2.4 configuration for ssl not working

On apache 2.4.25-3+deb9u4 (Debian obviously) the line mentioned by @Mr Roshan Pawar doesn't exists. You must create a symbolic link pointing to the module and enabling it in /etc/apache2/mods-enabled this way:

ln -s ../mods-available/socache_shmcb.load socache_shmcb.load

As @ericP said it's neccesary to restart your apache, depending on your distro/config it could be:

service apache2 restart
/etc/init.d/apache2 restart
systemctl restart apache2
apache2 -k restart

In Apache2.2 following line is uncommented in apache/conf/httpd.conf by default.

LoadModule socache_shmcb_module modules/mod_socache_shmcb.so

From Apache 2.4 above line is commented so remove the # sign before it.

This should work.