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

PHP Code:
    # yum install mod_ssl 
Once it is installed, make sure to restart the Apache service

PHP Code:
    # service httpd restart 
You should now be able to browse the website using https.