As far as i can tell you can either use another port than 8080 or better stop the server using that port. The command for the latter is:
This is somehow not written in the pset6 documentation, but in this video you can find the details by David: https://youtu.be/3dmp0ycKC5c
Hope that helps.
that means that some other process is already using the port (likely to be another instance of your server). terminating/killing that process should make the port available to use.
if this process is not another instance of your server, you probably should not do that and use some other port which is available. the command to kill a process using port 8080 ...