SSLEngine error on an Apache server

After you create the virtual host registration for port 443 during the certificate installation process, Apache refuses to start by displaying the "Invalid command 'SSLEngine'Error' error.

Cause
This is a common error in new instances installed by Apache. It is caused by the lack of the mod_ssl module which is responsible for creating and serving SSL connections.

The solution:
If your server is based on CentOS / RedHat Linux, run the following command:

sudo yum install mod_ssl

If you want to activate this module on a Debian solution (eg. Ubuntu) run the following command:

sudo a2enmod ssl

Then restart the Apache server:

sudo apachectl restart

If the server is started correctly you can verify the installation by running the command:

openssl s_client -connect domainname.tld:443

Replace domainname.tld - with the domain name for which you want to verify the certificate installation.