i2cWrite failed: 2

 I have an Arduino Mega  and a MPU 6050  HMC5883L 
I used example 
MPU6050_HMC5883L

connection VCC---3.3v
                  GND---GND
                  SCL---- SCL 20
                   SDA---SDA 21
i don´t know why  it is on the serial monitor always become  the message
i2cWrite failed: 2