useradd not found ?

I have one question here.

When I log into Linux server through SSH by the use of my personal account, then then use "su root" and passwd, why I can't create a new user account ?

I got error message like

useradd acb -d /home/www/html/acb
bash: useradd: command not found

It seems that I only can create a new user account by the use of "root" and "passwd" through SSH in the server. Something wrong ?

Please advise !