I want void loop to go through once

Projects General Guidance
15

so something like this then (with an active wait) ?

// BTS7960  pins
const byte  RPWM = 3;
const byte  LPWM = 6;
const byte  REN  = 4;
const byte  LEN  = 7;

// Limit Switch
const byte buttonPin = 12;

void setup() {
  pinMode(buttonPin, INPUT); // external pull down
  pinMode(RPWM, OUTPUT);
  pinMode(LPWM, OUTPUT);
  pinMode(LEN, OUTPUT);
  pinMode(REN, OUTPUT);
  digitalWrite(REN, HIGH); 
  digitalWrite(LEN, HIGH); 
}

void loop() {
  // wait for the limit switch to be activated
  while (digitalRead(12) == LOW); 

  // do your stuff once
  delay(500);
  digitalWrite(LEN, HIGH);
  digitalWrite(LPWM, HIGH);
  delay(4000);
  digitalWrite(LEN, LOW);
  digitalWrite(LPWM, LOW);
  delay(500);
  digitalWrite(13, HIGH);
  
// wait for the limit switch to be reset
  while (digitalRead(12) == HIGH); 
  delay(20); // poor's man anti-bounce
}

you don't use REN?
if you disable LEN, do you need to set the PWM to 0 too ?
13 was not set as an output, yet you do digitalWrite(13, HIGH);... that's not a good idea (activates the pull-up? ). is that just the LED built in or do you have something connected there?