You need to restart your Apache service for any changes you made to the config file to be effective. You must then test your Apache config for errors before restarting the service, as it could fail to start and disrupt the availability of the websites it is hosting.
Apache has a built-in feature to test the validity of the config file that you can use to catch errors or warnings. You can repeatedly check the configuration files until all the errors and warnings are gone before restarting the service.
You can use both apachectl and httpd tools to test your Apache configuration from the terminal.
$ sudo vi /etc/apache2/apache2.conf
$ sudo apachectl configtest AH00543: apache2: bad user name username Action 'configtest' failed. The Apache error log may have more information.
Use -t option instead of configtest if you're using httpd or apache and not apachectl
$ sudo apachectl configtest Syntax OK
$ sudo systemctl restart apache2 # Ubuntu, Debian, openSUSE and SLES $ sudo systemctl restart httpd # CentOS and Red Hat
Related: How to manage Apache service
Comment anonymously. Login not required.