error: 'cm' was not declared in this scope

I have no idea where this is coming from, i’m fairly new to programming. Any help (with notes on how you improved it) would be great!

/* HC-SR04 Sensor
https://www.dealextreme.com/p/hc-sr04-ultrasonic-sensor-distance-measuring-module-133696

This sketch reads a HC-SR04 ultrasonic rangefinder and returns the
distance to the closest object in range. To do this, it sends a pulse
to the sensor to initiate a reading, then listens for a pulse
to return. The length of the returning pulse is proportional to
the distance of the object from the sensor.

The circuit:

  • VCC connection of the sensor attached to +5V
  • GND connection of the sensor attached to ground
  • TRIG connection of the sensor attached to digital pin 2
  • ECHO connection of the sensor attached to digital pin 4

Original code for Ping))) example was created by David A. Mellis
Adapted for HC-SR04 by Tautvidas Sipavicius

This example code is in the public domain.
*/

int motorPin = 6;
int motor2Pin = 9;
const int trigPin = 2;
const int echoPin = 4;

void setup ( ) {
// initialize serial communication:
Serial.begin(9600);
pinMode(motorPin, OUTPUT);
pinMode(motor2Pin, OUTPUT);
}

void loop()
{
// establish variables for duration of the ping,
// and the distance result in inches and centimeters:
long duration, inches, cm;

// The sensor is triggered by a HIGH pulse of 10 or more microseconds.
// Give a short LOW pulse beforehand to ensure a clean HIGH pulse:
pinMode(trigPin, OUTPUT);
digitalWrite(trigPin, LOW);
delayMicroseconds(2);
digitalWrite(trigPin, HIGH);
delayMicroseconds(10);
digitalWrite(trigPin, LOW);

// Read the signal from the sensor: a HIGH pulse whose
// duration is the time (in microseconds) from the sending
// of the ping to the reception of its echo off of an object.
pinMode(echoPin, INPUT);
duration = pulseIn(echoPin, HIGH);

// convert the time into a distance
inches = microsecondsToInches(duration);
cm = microsecondsToCentimeters(duration);

Serial.print(inches);
Serial.print("in, ");
Serial.print(“cm”);
Serial.print(“cm”);
Serial.println();

delay(100);
}

long microsecondsToInches(long microseconds)
{
// According to Parallax’s datasheet for the PING))), there are
// 73.746 microseconds per inch (i.e. sound travels at 1130 feet per
// second). This gives the distance travelled by the ping, outbound
// and return, so we divide by 2 to get the distance of the obstacle.
// See: http://www.parallax.com/dl/docs/prod/acc/28015-PING-v1.3.pdf
return microseconds / 74 / 2;
}

long microsecondsToCentimeters(long microseconds)
{
// The speed of sound is 340 m/s or 29 microseconds per centimeter.
// The ping travels out and back, so to find the distance of the
// object we take half of the distance travelled.
return microseconds / 29 / 2;

if ( cm <=2){
//digitalWrite(motor, LOW);
digitalWrite(motor2Pin, LOW);
}
else
//digitalWrite(motor, HIGH);
digitalWrite(motorPin, HIGH);
digitalWrite(motor2Pin, HIGH);
delay(2000);
}

int motorPin = 6;
int motor2Pin = 9;
const int trigPin = 2;
const int echoPin = 4;
int cm; //<<<<<<<<<<Add this line

Did you add this to the code?

  if ( cm <=2){
    //digitalWrite(motor, LOW); 
    digitalWrite(motor2Pin, LOW);
  }
  else
    //digitalWrite(motor, HIGH);
    digitalWrite(motorPin, HIGH);
  digitalWrite(motor2Pin, HIGH);

What does this do?

FYI, LarryD is telling you some most basic features of the C language. You need to read a C tutorial before borrowing and hacking code from the internet. We can help with the error messages but YOU need to learn your ABC too.