It’s more about where your system saves the global node modules that’s the problem here.
Is it only newman that’s not working when you install that as a global module? Have you tried doing a global uninstall and install again? Did you quit the terminal after installing the module and then restarting it again, sometimes that’s not picked up. Does it work in a CMD window and not in the terminal you’re using?
I can basically list out questions all day long but I have no idea about your machine so i’m just guessing and making suggestions 