I have a plain Linux box RedHat 7.2
I want to add a user but i get a "command not found" message.

I tried "useradd" ,"adduser"

Thnx

You should use the whole path to the command

"/usr/sbin/useradd"

You really should use 'su -' instead of just 'su' to access another account, especially with root account. 'su -' will invoke bash as a login shell (I guess), so the profiles for root account will be loaded. On the other hand 'su' will retain the old profiles from the original user, which is a bit dangerous if you are not careful.