Invalid command “SSLEngine”, perhaps misspelled OR defined by a module not included.

We install a SSL certificate on a domain to secure the transaction carried out on a website but sometimes we receive an error message

“Invalid command “SSLEngine”, perhaps misspelled or defined by a module not included in the server configuration”

while browsing the website. The error message indicates that the module mod_ssl required to run SSL engine on a CentOS server is missing and needs to be installed.
Install the mod_ssl module using yum

#yum install mod_ssl

Once it is installed, make sure to restart the Apache service

#service httpd restart

You should now be able to browse the website using https.