Are you sure that you are root when trying? Because here is the thing. /sbin is "super-user bin" and it is a dir with programs only for root, therefor root should have it in $PATH but a regular user should not. Whoever you are you can check you $PATH with echo $PATH
Here is how you change $PATH.
Edit /root/.bashrc and add this line:
This line adds three sbin-dirs and at last the original path, to path. The reason we put it into the /root/.bashrc file is because now it will be updated every time you login as root.