how do i get the led to stay turn off at 11.9v

/*
  Blink

  Turns an LED on for one second, then off for one second, repeatedly.

  Most Arduinos have an on-board LED you can control. On the UNO, MEGA and ZERO
  it is attached to digital pin 13, on MKR1000 on pin 6. LED_BUILTIN is set to
  the correct LED pin independent of which board is used.
  If you want to know what pin the on-board LED is connected to on your Arduino
  model, check the Technical Specs of your board at:
  https://www.arduino.cc/en/Main/Products

  modified 8 May 2014
  by Scott Fitzgerald
  modified 2 Sep 2016
  by Arturo Guadalupi
  modified 8 Sep 2016
  by Colby Newman

  This example code is in the public domain.

  http://www.arduino.cc/en/Tutorial/Blink
*/

// the setup function runs once when you press reset or power the board
void setup() {
  // initialize digital pin LED_BUILTIN as an output.
  pinMode(LED_BUILTIN, OUTPUT);
   Serial.begin(9600);
}

// the loop function runs over and over again forever
void loop() {
   // read the input on analog pin 0:
  int sensorValue = analogRead(A0);
  // Convert the analog reading (which goes from 0 - 1023) to a voltage (0 - 12V):
  float voltage = sensorValue * (23.55 / 1023.0);
  // print out the value you read:
  Serial.println(voltage);
  digitalWrite(LED_BUILTIN, HIGH);   // turn the LED on (HIGH is the voltage level)
  delay(500);                       // wait for a second
  digitalWrite(LED_BUILTIN, LOW);    // turn the LED off by making the voltage LOW
  delay(1000);                       // wait for a second


}

Try a sensor value from 522 to 1023 and you will see that this comment and code are wrong:

  // Convert the analog reading (which goes from 0 - 1023) to a voltage (0 - 12V):
  float voltage = sensorValue * (23.55 / 1023.0);

The number 23.55 should probably be 12.

You need to learn about the "if" statement, curly brackets "{" and "}", and possibly the "else" statement.

I suggest that you start with
https://www.arduino.cc/reference/en/language/structure/control-structure/if/

One of the neat things about the Arduino ecosystem is that you get to do a lot of things for yourself, so I will not write code for you.

Good Luck!