Lost, please help!

Hi there! I got this project from Control Engineering. Basically, a tank controlled by Arduino Leonardo, with motor driver L298 and optical encoders which I have to read. I have been trying so solve this error with my code(I recieved from my teacher) and I cannot put it to an end. Please help!

#include <TimerOne.h>
#include <EnableInterrupt.h>
#include <Arduino.h>

#define NOT_AN_INTERRUPT -1
#define MOTOR_PIN_L_F 3
#define MOTOR_PIN_L_B 5
#define MOTOR_PIN_R_F 6
#define MOTOR_PIN_R_B 9

#define ENCODER_PIN_L 2
#define ENCODER_PIN_R 7

#define MAX_SPEED 160

#define START 8

//calibration
#define K_LEFT 0.004
#define K_RIGHT 0.006

//control parameters
#define GAIN_LEFT 1.254
#define GAIN_RIGHT 1.3254

#define THRESHOLD 0.005

//Timer t;

float LEFT_DISTANCE = 0.1;
float RIGHT_DISTANCE = 0.1;

volatile int impulse_left = 0;
volatile int impulse_right = 0;
float err_left;
float err_right;
float m_left;
float m_right;

void setup() {
Serial.begin(9600);
delay(10);
pinMode(ENCODER_PIN_L, INPUT);
pinMode(ENCODER_PIN_R, INPUT);

attachInterrupt(digitalPinToInterrupt(ENCODER_PIN_L), ISR_ENCODER_L, CHANGE);
attachInterrupt(digitalPinToInterrupt(ENCODER_PIN_R), ISR_ENCODER_R, CHANGE);

pinMode(MOTOR_PIN_L_F, OUTPUT);
pinMode(MOTOR_PIN_L_B, OUTPUT);
pinMode(MOTOR_PIN_R_F, OUTPUT);
pinMode(MOTOR_PIN_R_B, OUTPUT);

pinMode(START, INPUT_PULLUP);
}
void loop(){
void ISR_ENCODER_L() {
impulse_left++;
}

void ISR_ENCODER_R() {
impulse_right++;

void PID() {
err_left = abs(LEFT_DISTANCE) - impulse_left * K_LEFT;
err_right = abs(RIGHT_DISTANCE) - impulse_right * K_RIGHT;

if (err_left > THRESHOLD) {
m_left = GAIN_LEFT * err_left * 220 + 35;
}
else {
m_left = 0;
}

if (err_right > THRESHOLD) {
m_right = GAIN_RIGHT * err_right * 220 + 35;
}
else {
m_right = 0;
}

if (m_left > MAX_SPEED) {
m_left = MAX_SPEED;
}

if (m_right > MAX_SPEED) {
m_right = MAX_SPEED;
}

if (LEFT_DISTANCE > 0) {
analogWrite(MOTOR_PIN_L_F, m_left);
}
else {
analogWrite(MOTOR_PIN_L_B, m_left);
}

if (RIGHT_DISTANCE > 0) {
analogWrite(MOTOR_PIN_R_F, m_right);
}
else {
analogWrite(MOTOR_PIN_R_B, m_right);
}

}
}

if (digitalRead(START) == 0) {

}

}

The code you posted incorrectly either compiles, or it doesn't. If it doesn't, you can't expect us to guess why it doesn't. If it does, you can't expect us to guess what it actually does, and how that differs from what you want it to do.

void loop(){
  void ISR_ENCODER_L() {
    impulse_left++;
  }

  void ISR_ENCODER_R() {
    impulse_right++;


  void PID() {

I would guess that the code won't compile. There are more missing or misplaced curly brackets. Auto format (ctrl-t or Tools, Auto Format) would help to point them out. It would help if you put every curly bracket on its own line, too. Mismatches are easier to see that way.

void loop()
{
}

 void ISR_ENCODER_L()
{
   impulse_left++;
}