Can't start local server because port already in use

I'm working on this site: https://red-org.squarespace.com/ in dev mode but when I run this command:

squarespace-server https://red-org.squarespace.com --auth

I get the following output:

Apparently, that's saying my local server at that port is currently being used by another process.

My question is: how can I kill that process?

When I run

ps aux |grep 9000

to view all processes at that port, I get:

As you can see, each time I run

ps aux |grep 9000

, new PIDs are generated indicating (I think) that those are zombie processes. I try to kill the listed processes with

kill -9 <PID>

but it doesn't do anything, which seems to confirm that they are in fact zombie processes.

So how can I track down those parent processes and kill them so that I can run a Squarespace local server again — or is there another fix here? I'm running Mac Mojave 10.14.6.

Thanks.