I hit on this same problem, on an Ubuntu 17 VM.
Any idea how to disable the default application on port 80?
A list of commands to type will be nice, as it has been decades since I administered a Unix/Linux machine…
I used apt-get remove to remove apache*, nginx* and lighttpd, but it still says the port is in use.
EDIT: Got it to work. Turns out tcp6 was still listening on port 80. Rebooting the VM made it go away, and I was successful in installing Discourse. Thanks!