Velocity Measurement with Adafruit VL53L0X TOF (Time of Flight) Sensor

Other Hardware Sensors
4

Your code doesn't guarantee that timeA, timeB, distanceA or distanceB are even set!!

I'd suggest restructuring like this:

  if (measure.RangeMilliMeter < 2000){
    distanceA = measure.RangeMilliMeter;
    timeA = millis();
  
    delay (30);
  
    if (measure.RangeMilliMeter < 2000){
      distanceB = measure.RangeMilliMeter;
      timeB = millis();

      speed = (((distanceA-distanceB)/(timeB-timeA)));  // only perform this if all 4 variables actually defined
      return speed;
    }
  }
  return INVALID_READING ;