How to Fix “firewall-cmd: command not found” Error in ubuntu

sudo apt-get install firewalld

Next, start firewalld and enable it to auto-start at system boot, then check its status.

sudo systemctl start firewalld
sudo systemctl enable firewalld
sudo systemctl status firewalld

Communication between CephMonitors uses  port 6789 by default 

sudo firewall-cmd --zone=public --add-port=6789/tcp --permanent
sudo firewall-cmd --reload