enable_pin definition is missing in your code. Something like this may have been added just after the import section to solve the issue : enable_pin=18

A better solution is to remove the :
GPIO.setup(enable_pin, GPIO.OUT) and
GPIO.output(enable_pin, 1) lines, as this pin is not connected with the ULN2803A chip.

This pin configuraton was only provided in the adafruit tutorial for compatibility with other stepper chips.

Edit : this answer is working for the ULN2803A chip, question has been edited since for another chip.

enable_pin definition is missing in your code. Something like this may have been added just after the import section to solve the issue : enable_pin=18

A better solution is to remove the :
GPIO.setup(enable_pin, GPIO.OUT) and
GPIO.output(enable_pin, 1) lines, as this pin is not connected with the ULN2803A chip.

This pin configuraton was only provided in the adafruit tutorial for compatibility with other stepper chips.