How to resolve the error message below
Failed to issue method call: Unit firewalld.service is masked.

The main reason a service is masked is to prevent accidental starting or enabling of a serive.
For example firewalld can be masked with the following commad
#systemctl mask firewalld

If you ever want to start the firewalld service. you may get the error message
Failed to issue method call: Unit firewalld.service is masked.

To resolve this, unmask the firewall with the following command
#systemctl unmask firewalld

Source:http://theurbanpenguin.com/wp/?p=338