HelloWorld:4: error: 'LiquidCrystal_I2C' does not name a type
HelloWorld.ino: In function 'void setup()':
HelloWorld:8: error: 'lcd' was not declared in this scope

#include <Wire.h>
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27,16,2);  // set the LCD address to 0x20 for a 16 chars and 2 line display

void setup()
{
  lcd.init();                      // initialize the lcd
 
  // Print a message to the LCD.
  lcd.backlight();
  lcd.print("Hello, world!");
}

void loop()
{
}