PWM fan configuration

I just tried to enter the pwm fan configuration code on the printer.cfg file. I copied the code from the os.ratrig.com guide:

[fan]
pin: !fan_part_cooling_pin
cycle_time: 0.00004

but got the error " Pin 'fan_part_cooling_pin' is not a valid pin name on mcu 'mcu' " any help with how to configure this fan correctly would be greatly appreciated. Thank you