Can someone explain why "cells_creator" is not defined? (Expansion of dungeon_game.py)

import random # need this so we can get our starting points for player, monster and door
import os # still not familiar with this.  We will use a method from this module to clear the screen

def difficulty_selector():
    selection = input("What difficulty would you prefer? EASY, MEDIUM, HARD ").upper()
    if selection == "EASY":
        difficulty = 5
    if selection == "MEDIUM" or "HARD":
        difficulty = 10
    else:
        input("Sorry, that was not a valid entry: press ENTER to try again!")
    return difficulty


def cells_creator(difficulty):
    range_list = []
    for i in range(difficulty):
        range_list.append(i)
    index_tracker = 0
    map_layout = []
    complete = False
    while not complete:
        for item in range_list:
            map_layout.append(tuple((item, range_list[index_tracker])))
        index_tracker += 1
        if index_tracker > range_list[-1]:
            complete = True
    return map_layout

# Doesn't make sense to do this here moved below to execution area
# CELLS = cells_creator(difficulty)


def clear_screen():
    # I commented out the clear screen because I use eclipse and don't have TERM setup for development purposes- 
    print('clear screen called')
#     os.system('cls' if os.name == 'nt' else 'clear') # not very familiar with this but it will clear the screen.  Assuming this is one of those things that I'll learn as I get more experience


def get_locations():
    return random.sample(CELLS, 3) # takes 3 random samples, further in the code we unpack each sample and assign it to a variable


def move_player(player, move): # unpacks the tuple identifying the player's location and modifys the position based on the move
    x, y = player
    if move == "LEFT":
        x -= 1
    if move == "RIGHT":
        x += 1
    if move == "UP":
        y -= 1
    if move == "DOWN":
        y += 1
    return x, y


def monster_move(monster):
    x, y = monster
    available_monster_moves = []
    if monster[0] < 4:
        available_monster_moves.append(tuple((x + 1, y)))
    if monster[0] > 0:
        available_monster_moves.append(tuple((x - 1, y)))
    if monster[1] < 4:
        available_monster_moves.append(tuple((x, y + 1)))
    if monster[1] > 0:
        available_monster_moves.append(tuple((x, y - 1)))
    monster = random.choice(available_monster_moves)
    return monster


def start_screen():
    while True:
        print("Welcome to the dungeon!")
        decision = input("""Please enter START to start
Please enter HELP to see help """ ).upper()
        clear_screen()
        if decision == "START":
            break
        if decision == "HELP":
            input("""You must find the hidden door to exit the dungeon and win.
Don't let the hidden monster find you or you will lose.
You can only move: UP, DOWN, LEFT or RIGHT.
Press enter to go back to menu screen. """)
        clear_screen()



def get_moves(player): # lets the player know what direction they are allowed to move
    moves = ["LEFT", "RIGHT", "UP", "DOWN"]
    x, y = player
    if x == 0:
        moves.remove("LEFT")
    if x == 4:
        moves.remove("RIGHT")
    if y == 0:
        moves.remove("UP")
    if y == 4:
        moves.remove("DOWN")
    return moves


def draw_map(player, monster): # this function draws the map
    print(" _"*difficulty) # makes the top wall
    tile = "|{}"

    for cell in CELLS:
        x, y = cell
        # the brackets around -1 needed to be removed
        if x < difficulty -1:
            line_end = ""
            if cell == player:
                output = tile.format("X")
            elif cell == monster:
                output = tile.format("M")
            else:
                output = tile.format("_")
        else:
            line_end = "\n"
            if cell == player:
                output = tile.format("X|")
            else:
                output = tile.format("_|")
        print(output, end=line_end)


def game_loop():
    player, monster, door = get_locations() # here is where we unpack the tuples of locations and assign them to our main three variables
    playing = True


    while playing:
        clear_screen()
        draw_map(player, monster)
        valid_moves = get_moves(player)

        print("You're currently in room {}".format(player))  # gives the player their location on the map
        print("You can move {}".format(", ".join(valid_moves)))  # we use valid moves to show the player which direction they can move
        print("Enter QUIT to quit") # and a simple line that tells the player how to quit

        move = input("> ") # prompts the player to input a direction they want to move
        move = move.upper() # we want the players input to be ALL CAPS so it will match up with our code

        if move == 'QUIT': # simple if statement to make us break the loop if the player wants to quit
            print("\n ** See you next time **\n")
            break
        if move in valid_moves:
            player = move_player(player, move)

            monster_move(monster)

            if player == monster:
                print("\n ** Oh no! The monster got you! Better luck next time! **\n")
                playing = False
            if player == door:
                print("\n ** You escaped! Congratulations! **\n")
                playing = False

        else:
            input("\n ** Walls are hard!  Don't run into them! **\n")
    else:
        if input("Play again? [Y/N] ").lower() != "n":
            game_loop()
        else:
            input("Thanks for playing! Press enter to quit.")


clear_screen()
start_screen()
clear_screen()
# set difficulty in the global scope here
difficulty = difficulty_selector()
CELLS = cells_creator(difficulty)
clear_screen()
game_loop()