usb:0/140000/0/D is not available with usbMIDI

[Jibeji]

Hi,
Continuing my experiments on MIDI, I encounter another issue...
I am trying this simple code:

int potPin = A5;

const int numReadings = 20;

int readings[numReadings];      // the readings from the analog input
int readIndex = 0;              // the index of the current reading
int total = 0;                  // the running total
int average = 0;                // the average
int lastAverage;

void setup()
{
  pinMode(potPin, INPUT);
  usbMIDI.setHandleControlChange(OnControlChange);
  Serial.begin(9600);
  for (int thisReading = 0; thisReading < numReadings; thisReading++) {
    readings[thisReading] = 0;
  }
}

void loop()
{
  usbMIDI.read();
  //if there is a change, send and print it as a CC message
  if(average != lastAverage)
  {
    usbMIDI.sendControlChange(7, average, 16);
    Serial.print("Sent Volume: ");
    Serial.println(average);
  }
  lastAverage = average;
  delay(1);
}

void smoothing()
{
  total = total - readings[readIndex];
  readings[readIndex] = analogRead(potPin);
  total = total + readings[readIndex];
  readIndex = readIndex + 1;

  if (readIndex >= numReadings) {
    readIndex = 0;
  }
  average = total / numReadings / 8;
}

void OnControlChange(byte channel, byte control, byte value)
{
  Serial.println("Channel: ");
  Serial.print(channel);
  Serial.println("Control: ");
  Serial.print(control);
  Serial.println("Value: ");
  Serial.print(value);
}

But when I try to open the serial monitor, I have this error : usb:0/140000/0/D is not available

I have tried USB type as "MIDI" and "Serial + MIDI" with no success.