I think there's no less than 3 commands: useradd, adduser, and another whose name escapes me. Plus probably a "redhat-configure-user" command for the Red Hat platform.
Some of the above names are aliases, so there's fewer actual add user programs than commands.
It's common for install programs (such as RPM) to create user accounts, so you're not exactly doing anything exotic. Most likely you just need to make sure that the useradd command is in your path (or explicitly, a la "/sbin/useradd") and that you RTFMP'd (read the foolish man page) to get the options right.
And, of course, the "--help" option is you good friend in Linux
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.