Of course you are right. I made a ridiculous mistake, the plugin works perfectly. Thanks!
I would, however, need some advice. I need that when the plugin shutdowns the server, an mqtt command to be executed before. The solution I found so far was to copy the nut-notify script to /boot/config, modify it as I want, and every time the server is restarted, it is copied back to the /usr/sbin/ folder. Is there a more elegant solution to this problem?
Thanks!