Setting up the mosquitto password file

[badflash]

I'm running into some weird stuff. I am following a tutorial on setting up mosquitto here:
https://www.youtube.com/watch?v=72u6gIkeqUc

But I get this weird error when I try to set up the password. Any ideas?

[UnborN]

mosquitto_passwd -U password.txt
-U should be capital (source)

[badflash]

That did something, but I still can't get it working. I'll try uninstalling and starting over.