If you want to install an SSL certificate on an Apache server, you must install the mod_ssl.so module in advance to enable SSL encryption. You can run the httpd -M | grep 'ssl' command to check whether the mod_ssl.so module is successfully installed. If the module is not installed, run the yum install -y mod_ssl command to install the module. After the module is installed, the ssl.conf file appears in the etc/httpd/conf.d directory.