#include "DigiKeyboard.h"

int ldr = 0;

void setup() {
  Serial.begin(9600);

}

void loop() {
  ldr = analogRead(1);
  if (ldr > NUMERO DE REFERENCIA) {
    DigiKeyboard.sendKeyStroke(44);
    delay(30);
  }
  else { return; }
}

Line 25: error: DigiKeyboard.h: No such file or directory
In function 'void setup()':
Line 6: error: 'Serial' was not declared in this scope
compilation terminated due to -Wfatal-errors.