So I am working on a ground surveillance robot using Arduino for coding the Gps, motors and ultrasonic sensors and coding in raspberry Pi for facial recognition but now what I want to do is when my robot has gotten to its final waypoint and has stopped it should excute the python file that does the facial recognition by calling it in the Arduino sketch but I don't know how to do it would be nice if someone could please help me out I appreciate your help in advance thank you
You can't make the Arduino just run the Python script on the Pi. Instead you will have to run some software (probably of your own devising) on the Pi that listens for instructions from the Arduino.
The simplest way is probably to modify your existing Python script so that it runs all the time but sits idling waiting for instructions through the serial port (using pySerial). When it gets the instruction to perform the task you need it then goes off and runs the main bulk of its code. When that is done it can then return the results back through the serial port to the Arduino (if you need such results returning).
How you arrange those instructions to get it to do things is entirely up to you.