Django - Python - Djaunty Rent-a-Bike error

I got stuck at step 14 in the Djaunty Rent-a-Bike project found here: https://www.codecademy.com/paths/build-python-web-apps-with-django/tracks/data-in-django/modules/django-models-and-databases/projects/djaunty-rent-a-bike

When doing the makemigrations step, i get the error " NameError: name ‘TANDEM’ is not defined". I cannot get why this happens as Tandem is clearily defined and in the right spot as well, above the function calling it. I have triple checked with the get-unstuck video, with other peoples’ code and i dont find anything wrong on my part.

Here is my code:

from django.db import models
import datetime

BASE_PRICE = 25.00
TANDEM_SURCHARGE = 15.00
ELECTRIC_SURCHARGE = 25.00

# Create your models here.
class Bike(models.Model):
  STANDARD = "ST"
  TΑΝDEM = "TA"
  ELECTRIC = "EL"

  BIKE_TYPE_CHOICES = [
    (STANDARD, "Standard"),
    (TANDEM, "Tandem"),
    (ELECTRIC, "Electric"),
  ]

  bike_type = models.CharField(max_length=2, choices=BIKE_TYPE_CHOICES, default=STANDARD)
  color = models.CharField(max_length=10, default="")
 

  def __str__(self):
      return self.bike_type +"-"+self.color

class Renter(models.Model):
  first_name=models.CharField(max_length=30)
  last_name=models.CharField(max_lenght=30)
  phone = models.IntegerField(max_length=15)
  vip_num = models.IntegerField(default=0)

  def __str__(self):
      return self.first_name + self.last_name+ " - #" + self.phone

class Rental(models.Model):
  bike = models.ForeigKey(Bike, on_delete=models.CASCADE)
  renter = models.ForeignKey(Renter, on_delete=models.CASCADE)
  date = models.DateField(default=datetime.date.today)
  price = models.FloatField(default=0.0)

  def calc_price(self):
    curr_price = BASE_PRICE

    if self.bike.bike_type == "TA":
      curr_price+=TANDEM_SURCHARGE
    elif self.bike.bike_type == "EL":
      curr_price+=ELECTRIC_SURCHARGE
    elif self.renter.vip_num > 0:
      curr_price *= 0.8 
  
  self.price = curr_price