If you’re using Putty to connect with ssh, then presumably the server has the ssh server already installed.
Have you been able to determine which command in the script is the one actually failing? If you can determine that, then you can debug the command itself.
Unfortunately, it’s unclear from the issue @eXTric posted whether that issue was ever resolved or not.
Is the behavior you’re seeing the same that was posted in that issue? If it is something else, can you paste it here?
Also notable: could be user error (I’ll keep tinkering) but I haven’t been able to get mailinabox logs to record anything. Hostinger’s requested them and I can’t get logs thus far, just the file creation but no logs inside. But I’m by no means an expert and will keep trying.
Those two screenshots are showing me a series of hostname resolution failures. Is your DNS setup correctly? Can you resolve any of hostnames it’s complaining about? Determine if you can resolve those hosts using something like host, nslookup or dig.