Find out more about how pi-topOS uses port 80
pi-topOS (June 2021 onwards) comes with an application called 'pt-web-portal', which offers a web-based interface for the majority of pi-topOS's custom GUI applications.
This application runs a web server on port 80 - the standard port for web traffic on a computer. This means that if you are on the same network as your pi-top device / Raspberry Pi, and you access the IP address or hostname of the device via a web browser, you will be able to access the web portal.
Ports cannot be shared, and so this means that this port is taken as a "first-class citizen" of the underlying operating system.
If you wish to run something on port 80, you will need to deactivate 'pt-web-portal'. However, it is strongly recommended that you choose an alternative port for your application and leave 'pt-web-portal' intact.
To find out more about pi-topOS's web portal (including how to turn it off), check the OS documentation.