I added export PATH=./:~/:~/bin/:/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games:/sbin:/usr/sbin to /root/.bashrc
That fixed the problem. useradd worked. I added another user account.
Thank you, Arjen. I am grateful you pointed out the importance of /usr/sbin/ to me.
I also saw the info. you added at /usr/share/solydxk/info Nice.
I did not know until now that su could take any arguments. I will further investigate this fact.
I have the new user account. I set the password as superuser, and saw no output from passwd. However, the new user was unable to log in using the password I gave passwd. I will have another look at this after lunch.