Cannot start a service, getting "Failed to start unit .. is masked" error

Issue

• Cannot start or enable a service, getting "Failed to .. is masked" message.
• Cannot start or enable a systemd unit, getting "Failed to .. is masked" message.

Example when trying to start the httpd.service unit:

# systemctl start httpd
Failed to start httpd.service: Unit httpd.service is masked.

Example when trying to enable the httpd.service unit:

# systemctl enable httpd
Failed to enable unit: Unit file /etc/systemd/system/httpd.service is masked.