"command not found", means just that. It couldn't find the command "startx".
So, maybe you should check your user's PATH:
If the path to startx isn't there, add it. Don't know where startx is? Then:
For me it is /usr/X11R6/bin/startx, so I need /usr/X11R6/bin/ in my PATH and I add it and export it like this: