when you do a su to root, you might not have your PATH set up correctly.
Depending on your distribution, users (including root) probably have their .bash_profile (runs at login) set up, but not there .bashrc (runs at new shell).
Simply append the sbin dirs to your PATH in .bashrc
#vi .bashrc
export PATH=/usr/local/sbin:/usr/sbin:/sbin
or for your specific case, just call useradd with its full path name
# /usr/sbin/useradd -d newuser /home/users
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