A service fails to start with the message:
# systemctl start service_name.service
Unable to start service_name: systemctl start service_name.service Failed to start service_name.service: Unit service_name.service is masked.
The service is masked.
Connect to the Plesk server via SSH.
Unmask the masked service:
# systemctl unmask service_name
Start the service:
# systemctl start service_name