the SITL setup page says to run sim_vehicle.py . It took quite a while to track this down, knowing nothing about Ardupilot or SITL. I finally found the command sim_vehicle.sh in the arduplane/Tools/autotest durectory, which the setup page does have you add to your PATH variable. Can someone please update this to spare anyone else the pain and frustration 
In case you didn’t catch the problem sim_vehicle.sh not sim_vehicle.py
Sorry, we didn’t handle the transition from sim_vehicle.sh to sim_vehicle.py too well.
If you are working on the master branch of ArduPilot you will find sim_vehicle.sh will not exist but sim_vehicle.py does.
Which branch are you working on that you have the “old” sim_vehicle.sh?
Peter