TCP/IP ports range from 1 to 65535. Abyss Web Server reports a listening problem on a port when one of the following situations is happening:
Another instance of Abyss Web Server is already running on your computer.
The user has not the privilege to use the port: By design, Mac OS X/macOS allows the applications to only use ports ranging from 1024 to 65535.
Using ports 1 to 1023 is generally only possible when authorized by an administrator of the computer, except as stated in the note "Special ports on recent macOS versions".
To activate the priviledged, run Abyss Web Server, select Activate ports below 1024 in the Server menu, and ask an administrator of the computer (you or another person) to authenticate himself/herself.
Note that ports activations is a one-time operation. It remains valid until you reinstall Abyss Web Server or until you move the executable file to another folder.
The port is already used by another application: Try stopping the other applications running on your computer to know which one is already using this port. More specifically, port 80 is usually used by other web servers. So if Abyss Web Server cannot listen to this port, there are chances that another web server is already running. Mac OS X/macOs comes with a web sharing feature that, when enabled, uses port 80. To disable it and to make this port available for Abyss Web Server, open System Preferences, select Sharing and uncheck Personal Web Sharing.
An alternative solution would be to configure Abyss Web Server to use another web host or console port. For more information about changing the host port, refer to "Hosts Management".
If you have to change the console port, select Settings… in the main window, enter a new value in the Port field , press OK and confirm starting the server.