serial 1 was not declared in the scope #include &lt;LiquidCrystal.h&gt;
LiquidCrystal lcd(2, 3, 4, 5, 6, 7);
#include &lt;SoftwareSerial.h&gt;


char Mob[20]="+923319020922";
String stringGPS = "";
String stringGPS1 = "";
char c=0;
int t=0,i;
int buzzer = 9;
int buttonState = 0;

SoftwareSerial mySerial(10, 11); // RX, TX

//const int buttonPin1 =9 ;
int buttonState1 = 0;
int helmetstate=0;
const int relay=8;
char inByte=0;

void setup() 
{
  lcd.begin(20, 4); 
  lcd.setCursor(0, 0);
  pinMode(relay, OUTPUT);
  pinMode(buzzer, OUTPUT);
  Serial.begin(9600);
  Serial1.begin(9600);
  Serial2.begin(9600);
  digitalWrite(relay, LOW);
  delay(15000); 
  lcd.print("Config modem...."); 

  Config(); 

  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("Smart helmet      ");
}

void loop()
{
  if(Serial2.available())
  {
    inByte = Serial2.read();  
    //Serial.write(inByte);
    if(inByte=='A'||inByte=='B'||inByte=='C'){
    inByte=0;
    SMS(Mob);

    while(Serial2.available()&gt;0)
    {
      i = Serial2.read();
    }
  }

  if(inByte=='D')
  {
    digitalWrite(relay, LOW);
    inByte=0;
  }

  if(inByte=='E')
  {
    digitalWrite(relay, HIGH);
    inByte=0;
  }
  else 
  {
  }
} 

 //buttonState = digitalRead(buttonPin);  
 // if (buttonState == LOW) {
 //Serial.print("ok");
 //lcd.clear();
 // lcd.setCursor(0, 0);
 //lcd.print("Sending msg       ");
 // lcd.setCursor(0, 0);
 //lcd.print("Smart helmet      ");
 // checkGPS();
 // SMS(Mob);

 //helmet();
 checkGPS(); 
 lcd.setCursor(0, 1);
 lcd.print(stringGPS1); 
 //Serial.println(stringGPS1);
}

/*
void helmet()
{
  buttonState1 = digitalRead(buttonPin1);
  if (buttonState1 == HIGH) 
  {
    lcd.clear();
    lcd.setCursor(0, 0);
    lcd.print("SWITCH ON  .......    "); 
    digitalWrite(relay, HIGH);
  }
  else 
  {
    lcd.clear();
    lcd.setCursor(0, 0);
    lcd.print("SWITCH OFF .......    "); 
    digitalWrite(relay, LOW);
  }
}
*/

void checkGPS()
{
  // Serial.print("check gps");
  // while (Serial.available()) 
  // {
  if (Serial.available()&gt;0)
  {
   // c = Serial.read();        
   // stringGPS  = c;
   //Serial.print(c);
   while (c != 'R') 
   {
     c = Serial.read();
     // Serial.print(c);
   }
    // if (c== 'R')
    // {
    //   Serial.print(c );
         while(t!=41)
         {
           if (Serial.available())
           {
             c = Serial.read(); 
             stringGPS += c;
             t++;
             //Serial.print(stringGPS );
          }
  }
  stringGPS1= stringGPS.substring(15, 41);
  // Serial.println(stringGPS1);
  t=0;
  c=0;
  stringGPS = "";
    // }
  }
  }

void SMS(char Mob[20])
{
   digitalWrite(buzzer, HIGH);
   lcd.setCursor(0, 0);
   lcd.print("Sending msg       ");
   Serial1.print("AT+CMGS=\"");     // send the SMS number
   Serial1.print(Mob);/// Send mobile number
   Serial1.print("\"\r");
   delay(4000);  
   Serial1.print(stringGPS1);// send the SMS Data
   Serial1.print("\r");
   delay(2000);
   Serial1.write(0x1A);  ///Ctrl+Z  
   delay(1000);
   Serial1.write(0x0A);  ///Carrige return   
   delay(1000);
   Serial1.write(0x0D);    ///line feed 
   delay(1000);
   stringGPS1 = "";
   lcd.setCursor(0, 0);
   lcd.print("Smart helmet      ");
   digitalWrite(buzzer, LOW);    
}

void Config()/////configuration for sim900 
{
  delay(4000);
  Serial1.print("ATE0\r"); ////Disable Echo
  delay(4000);
  Serial1.print("AT\r"); ///checking echo
  delay(4000);
  Serial1.print("AT+CMGF=1\r");///msg format
  delay(4000);     
}


when i run this code it give me error that serial1 was not decleared in the scope
The only SoftwareSerial you a declaring is SoftwareSerial mySerial(10, 11); // RX, TX

(And you you didn't use it afterwards).

If the hardware serials Serial1, Serial2 or Serial3 are available
(and already declared in the arduino.h header depends on the board you are using.