If you are behind a hardware router/firewall, there really is no need to run a software firewall on your systems. In fact, if you are attempting to connect from one system to another on your local network, a firewall might be stopping you from making the connection. And of course, computers are famous for not telling us "whats wrong".

So on my new install of a Linux OS (Ubuntu) I Googled "how to disable firewall".  I found this:

The latest version of Ubuntu comes with ufw (now it is the default firewall configuration tool for Ubuntu). It is developed to ease iptables firewall configuration, ufw provides a user friendly way to create an IPv4 or IPv6 host-based firewall. To disable ufw, enter:

sudo ufw disable