Hello ratnamraj raj,
As long as you follow a standard install, setting up a fresh server and re-install Discourse is often the best way to make things work instead of trying to fix broken or misconfigured configurations.
If your port 80 is already used, it means that there’s software using it. Either the server you set up is pre-configured with a web server or similar software or you previously installed things on your server prior to installing Discourse.
What is your server’s provider? What version of Linux are you using?
Did you install Discourse following the official guide straight after creating the server?