su (sudo) command not found Galaxy S Vibrant

I was wondering if any of you has seen this behavior: I have an unrooted *Galaxy S Vibrant* and I was trying to root it permanently with z4root, unsuccessfully. While I try to do it permantly it seems to do it, but when I try to run *Titanium Backup* or any app that requires root permissions, it won't gain privileges. So, I installed a terminal app, and when I tried to run the su command it displays a command not found error. My question is, what is missing from the OS? Any Idea? I know that you must have to be root in order to run certains commands but in that case it should display another error message?