1996 Mercury Sable Wont move

Try inspecting the shift linkages

P0500 OBD-II Trouble Code
Technical Description Vehicle Speed Sensor Malfunction

What does that mean?Basically this means that the vehicle's speed as read by the Vehicle Speed Sensor (VSS) is not working properly.

SymptomsSymptoms may include:
loss of anti-lock brakes
the "anti-lock" or "brake" warning lamps on the dash may be lit
the speedometer or odometer may not work properly (or at all)
your vehicle's RPM limiter may be decreased
the shifting of an automatic transmission may become erratic
other symptoms may also be present
CausesA code P0500 may mean that one or more of the following has happened:

The Vehicle Speed Sensor (VSS) is not reading (functioning) properly
There is a broken/frayed wire leading to the vehicle speed sensor