Failed to start service: Unit is masked.

Symptoms

You may see an error like the following when trying to start a service.

Failed to start pdns.service: Unit is masked.

Description 

This is due to the service (pdns in this example) being masked. Masking a service prevents it from starting.

Workaround

The service can be unmasked with the following command, where SERVICE is replaced with the actual service.

systemctl unmask SERVICE