"Serial" does not name a type -> How to solve this problem? I'm trying my best to solve this problem but it didn't changed

The red line was drawn on the last Serial.println */

int cds = A0;
int led = 12;
int trig1 = 2;      // Left Trig
int trig2 = 3;      // Right Trig
int trig3 = 4;      // Front Trig
int trig4 = 5;      // Upside Trig
int echo1 = 6;      // Left Echo
int echo2 = 7;      // Right Echo
int echo3 = 8;      // Front Echo
int echo4 = 9;      // Upside Echo
int vib1 = 10;      // Left vib
int vib2 = 11;      // Right vib 


void setup() {
        pinMode(led, OUTPUT);

        pinMode(trig1, OUTPUT);
        pinMode(trig2, OUTPUT);
        pinMode(trig3, OUTPUT);
        pinMode(trig4, OUTPUT);
        pinMode(echo1, INPUT);
        pinMode(echo2, INPUT);
        pinMode(echo3, INPUT);
        pinMode(echo4, INPUT);
        pinMode(vib1, OUTPUT);
        pinMode(vib2, OUTPUT);

        Serial.begin(9600);   
}


void loop(){
  long duration1, value1;
  long duration2, value2;
  long duration3, value3;
  long duration4, value4;

  // Ultra Sonic Sensor Part1
  digitalWrite(trig1,LOW);                           // Trigger Output
  delayMicroseconds(2);
  digitalWrite(trig1,HIGH);
  delayMicroseconds(5);
  digitalWrite(trig1,LOW);
  duration1 = pulseIn(echo1,HIGH);                   // Read Pulse Duration
  value1 = microsecondsToCentimeters(duration1);     // Count Distance
  delay(10);

  // Ultra Sonic Sensor Part2
  digitalWrite(trig2,LOW);   
  delayMicroseconds(2);       
  digitalWrite(trig2,HIGH);    
  delayMicroseconds(5);
  digitalWrite(trig2,LOW);
  duration2 = pulseIn(echo2,HIGH);
  value2 = microsecondsToCentimeters(duration2);
  delay(10);

  // Ultra Sonic Sensor Part3
  digitalWrite(trig3,LOW);
  delayMicroseconds(2);
  digitalWrite(trig3,HIGH);
  delayMicroseconds(5);
  digitalWrite(trig3,LOW);
  duration3 = pulseIn(echo3,HIGH);
  value3 = microsecondsToCentimeters(duration3);
  delay(10);

  // Ultra Sonic Sensor Part4
  digitalWrite(trig4,LOW);
  delayMicroseconds(2);
  digitalWrite(trig4,HIGH);
  delayMicroseconds(5);
  digitalWrite(trig4,LOW);
  duration4 = pulseIn(echo4,HIGH);
  value4 =  microsecondsToCentimeters(duration4);
  delay(10);

  //Commend Part (Vibration motor)

  // Print Out Value  
  Serial.print(" dist1 =");
  Serial.print(value1);
  Serial.print("(cm)");
  Serial.print(" dist2 =");
  Serial.print(value2);
  Serial.print("(cm)");
  Serial.print(" dist3 =");
  Serial.print(value3);
  Serial.print("(cm)");
  Serial.print(" dist4 =");
  Serial.print(value4);
  Serial.print("(cm)");
  delay(10);
  //

  // Initialize PIN Mode
  digitalWrite(vib1,HIGH);
  digitalWrite(vib2,HIGH);

  // 진동모터1,2,3
  if (  value4&lt;60  or  value3&lt;60)
  digitalWrite(vib1,LOW);
  digitalWrite(vib2,LOW);

  delay(100);

  digitalWrite(vib1,HIGH);
  digitalWrite(vib2,HIGH);

  if (0&lt; value1 &amp;&amp; value1&lt;60)
  digitalWrite(vib1,LOW);

  delay(100);

  digitalWrite(vib1,HIGH);

  if (0&lt; value2 &amp;&amp; value2&lt;60)
  digitalWrite(vib1,LOW);

  delay(100);

  digitalWrite(vib1,HIGH);
}

// Function of Count Distance

long microsecondsToCentimeters(long microseconds)
{
  return microseconds/29/2; 
}

        int cdsValue = analogRead(cds);

        Serial.print("cds =  ");
        Serial.println(cdsValue);

        if (cdsValue &lt; 38 ) {
                digitalWrite(led, HIGH);
        } else {
                digitalWrite(led, LOW);
        }
Your code would be a lot easier to read and find mistakes in if you formatted it. I see that you did that in the IDE, but in your question, you need to highlight all of the code, then click the "{}" button to make it look like code in your question.

When I pasted your code into my IDE and auto-formatted it, the answer was immediately clear: after the function microsecondsToCentimeters() you have written some code that is not inside any function. C and C++ don't allow that. The error message wasn't very helpful - the compilers easily get confused by our mistakes - but in this case, looking at the formatted code made it clear.

All of the executable code in a C or C++ program (Arduino IDE uses C++) must be inside a typed and named function. Since that is what the compiler was looking for, it seems to have interpreted the word "Serial" as a function type, but no type was defined with that name.

I'm not sure where you intended the lines from the end of long microsecondsToCentimeters() to go, but they need to be inside of a function, somewhere.