'input' was not declared in this scope

void loop() {
while (!mpuInterrupt && fifoCount < packetSize)
{
//no mpu data - performing PID calculations and output to motors
pid.Compute();

//Print the value of Input and Output on serial monitor to check how it is working.
Serial.print(input); Serial.print(" =>"); Serial.println(output);

if (input>150 && input<200){//If the Bot is falling

if (output>0) //Falling towards front
Forward(); //Rotate the wheels forward
else if (output<0) //Falling towards back
Reverse(); //Rotate the wheels backward

The problem is in the code you didn't post.

Should that be Input (note capital I) and Output (capital O). Remember that C++ is case sensitive. Those are the variables in the PID library examples.