the "su - dirtsand" commando is used to switch to the dirtsand user so you don't run it as root. The authentication failure means that the password you type in it is incorrect and the switch fails. No you should not ignore that error.
The errors we told you to ignore earlier were from dirtsand because there wasn't a configuration file yet and therefore cannot run yet as a fully functional dirtsand server. The authentication error means that the command failed.
PeterC, your questions are mostly related to Linux in general. You might have better luck (and more answers) if you asked on the Ubuntu forums, for instance, or on a Linux development forum. We can help you, but we can't teach you Linux by forum!