You will need to enable the UART serial interface on the Raspberry Pi. Like most interfaces, this is disabled on the Pi by default. Open the Raspberry Pi configuration menu with the following command:

sudo raspi-config

Under Interfacing Options choose Serial.

You will be asked, "Would you like a login shell to be accessible over serial?" Select No.

When you see the question, "Would you like the serial port hardware to be enabled?" Choose Yes.

Finally, reboot the Raspberry Pi:

sudo reboot

You can find more information on enabling the UART serial interface on Adafruit's website.