This error occurs when you run an app on 3000 port of server which was already occupied by other process. Then you need to kill the running process and run your desired process.
If you find this error in while running RoR application on server, then Type this in your terminal to find out the PID of the process that’s using the 3000 port:
It will show process number in second position like
ruby 6255 adam 4u IPv4
here 6255 is process number.
$kill -9 <process number>