After logging in as root, type "which id", which should show it as /usr/bin/id. Now type "echo $PATH", which should show root's path statement, which should include at least /bin:/sbin:/usr/bin:/usr/sbin AFAIK.
If "which id" didn't show /usr/bin/id, then you're missing the command, else
If "which id" shows /usr/bin/id, but executing it as "id" gives an error, make sure /usr/bin is in your PATH statement, else
if "which id" shows /usr/bin/id, and /usr/bin *is* in your PATH statement, you've got a problem of a different order.