ignore

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Do you have the library "DS3231" ?

So if you go into your directories, such as the 'arduino' directory, and go into the 'libraries' directory (folder), then do you have a directory called 'DS3231'? And within it...do are there files such as DS3231.c and DS3231.h etc?

yes I have the library

Which Arduino? You need to use the pins in the line that matches your board.

// To use the hardware I2C (TWI) interface of the Arduino you must connect
// the pins as follows:
//
// Arduino Uno/2009:
// ----------------------
 //DS3231  SDA pin   -> Arduino Analog 4 or the dedicated SDA pin
  //       SCL pin   -> Arduino Analog 5 or the dedicated SCL pin
//
// Arduino Leonardo:
// ----------------------
// DS3231:  SDA pin   -> Arduino Digital 2 or the dedicated SDA pin
//          SCL pin   -> Arduino Digital 3 or the dedicated SCL pin
//
// Arduino Mega:
// ----------------------
// DS3231:  SDA pin   -> Arduino Digital 20 (SDA) or the dedicated SDA pin
//          SCL pin   -> Arduino Digital 21 (SCL) or the dedicated SCL pin
//
// Arduino Due:
// ----------------------
// DS3231:  SDA pin   -> Arduino Digital 20 (SDA) or the dedicated SDA1 (Digital 70) pin
//          SCL pin   -> Arduino Digital 21 (SCL) or the dedicated SCL1 (Digital 71) pin

If UNO:
DS3231 rtc(A4, A5);

Why did you delete your question? That is incredibly rude to do after you've got responses.