Getting "Unknown" error in Terminal?

Please post a URL pointer to the directions used here; to whatever was entered.

Minimally, something appears to have changed your prompt string.

What else might have happened here is not clear.

That's not an error, and you don't need to do anything about it.

It is definitely an error

No, it definitely is not. It's the shell prompt, showing the result of a reverse DNS lookup on the IP address of the network interface connected to the default route. Unless you're going to become a regular shell user, there's no reason to change it. If you want something to worry about, worry about what this command file is going to do to your system, which could be anything at all.

Link Davis is correct. It is not an error.

By default the shell does a Reverse DNS lookup and the string returned is what the shell displays as part of your prompt.

The fact that your DNS server does not have a name associated with your IP address is common. However in this case your DNS server decided to return unknown-xx.xx.xx.xx.xx.xx is different, but still not an error.

If you were going to be a heavy Terminal user, there are thing you can do to ignore the DNS server response, but since this is a one off Terminal use, just accept that this is not an error from people that spend hours every day working in the Terminal/Unix environment.

Somebody really needs to create a Minecraft installer for OS X, because you're only the most recent person to become entangled by the manual installation process.

I'm not going to watch a YouTube video to see what's gone weird with it; sorry. FWIW, There are text-based Minecraft installation and setup discussions around — I've seen a few discussed in previous reports of Minecraft server installation problems here. Those are generally better than the videos, as you have commands you can copy. Mistakes at the command line can lead to problems far worse than a prompt string change, too; data loss is possible, with certain sorts of commands.

Beyond the Minecraft server installation process, one of the prerequisites for the most common Minecraft server — Oracle Java — needs to be managed and kept current. Past versions of Oracle Java have allowed security to be breached, and there's no reason to expect that the most current version won't eventually be breached. Due to this history of vulnerabilities, the Apple OS X XProtect mechanisms will occasionally be updated to block Java when these vulnerabilities are reported. Put another way, you might find your Minecraft server unexpectedly taken offline when there's a Java security vulnerability loose.

As for the prompt string, here's a write-up on the prompt-changing bits.