FireWall in Centos 7 masked

FireWall in Centos 7 masked
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