Your script states that only an operator (from mIRC point of view) can use the function you made, either they are an operator or they are not (again, from mIRC point of view).
From what I've read about twitch, it's not acting as a real IRC server and that has consequences.
A simple test is to execute in your mIRC's editbox:
//if (<nickname> isop #yourchannel) echo -a yes
And if that nickname is an operator from mIRC's point of view, you'll have the echo, otherwise mIRC doesn't recognize that nickname as an operator, which would be an error caused by twitch not being a real IRC server, or maybe just a fail from you where the nickname was in fact not an operator
