There's a difference between the getUser($chan,$nick) you used and
$getUser($chan,$nick)
Without the dollar, mIRC thinks you're trying to execute an alias name getUser($chan,$nick) without passing any parameters to it.
Unless there's something that twitch servers handle differently than mIRC, I can't see why an IRC server doesn't recognize NOTICE as a 'real' command that lets irc clients send non-query communications to each other, or why asking the server to do a /whois on someone would be related to whispering something to them.
I've been explaining things as if twitch were just an interest-group happening to also be using mirc. But from looking at a website describing the twitch interface, it appears that any advice you need related to your mirc's interaction with a twitch server might need to come from another twitch user. However, the reason why getUser($chan,$nick) without the dollar is a syntax error is an internal mirc issue that has nothing to do with using a twitch server or a standard server.
If you go somewhere like irc.freenode.net you'll see that "//notice $me test" is a real command. Whether it's also real at a twitch server is something else.
