On dungeon_game.py I keep getting : TypeError: can only join an iterable

Here is the particular block of code that the error is pointing at. 

while True:
    valid_moves = get_moves(player)
    clear_screen()
    print("Welcome to the dungeon!")
    print("You are currently in room {}".format(player))  # fill with player position
    print("you can move {}".format(", ".join(valid_moves)))  # fill with available moves.
    print("Enter QUIT to quit")

here is all of the code so far. 

"""


======================================================================================================================

Notes: random.sample function , to make sure that our random selections selection does not 
overlap. by doing that instead of: monster = None
                                   door = None
                                   player = None
                                   return monster, door, player
you can: return random.sample(CELLS, 3), cells variable is the iterable. 

Note: too many values to unpack error means you have extra elements that needs to be packed , use return 
to return the extra variables. 

======================================================================================================================

Program Needs: 
# draw grid
# pick a random location for the player.
# pick random location for exit door.
# pick random location for the monster.
# draw player in the grid.
# take input for movement.
# move player, unless invalid move (past edges of grid)
# check for win/loss.
# clear scree and redraw grid.

======================================================================================================================


"""


import os
import random

# to create grid, grid is 5*5.
CELLS = [(0, 0), (1, 0), (2, 0), (3, 0), (4, 0),
         (0, 1), (1, 1), (2, 1), (3, 1), (4, 1),
         (0, 2), (1, 2), (2, 2), (3, 2), (4, 2),
         (0, 3), (1, 3), (2, 3), (3, 3), (4, 3),
         (0, 4), (1, 4), (2, 4), (3, 4), (4, 4)]


def clear_screen():
    os.system('cls' if os.name == 'nt' else 'clear')


def get_locations():
    return random.sample(CELLS, 3)


def move_player(player, move):
    x, y = player
    if move == "LEFT":  # if move == LEFT, x-1.
        X -= 1
    if move == "RIGHT":  # if move == RIGHT, x+1.
        x += 1
    if move == "UP":  # if move == UP, y-1.
        y -= 1
    if move == "DOWN":  # if move == DOWN, y+1.
        y += 1

    return x, y


def get_moves(player):
    moves = ["LEFT", "RIGHT", "UP", "DOWN"]  # get player's location.
    x, y = player

    if x == 0:  # if player's  x == 0, they cannot move left.
        moves.remove("LEFT")
    if x == 4:  # if player's x == 4, they cannot move right.
        moves.remove("RIGHT")
    if y == 0:  # if player's y ==0, they cannot move up.
        moves.remove("UP")
    if y == 4:  # if player's y == 4, they cannot move down.
        moves.remove("DOWN")
    return x, y
    return moves

# put outside of the while loop, because you do not want to relocate each time.
monster, door, player = get_locations()


while True:
    valid_moves = get_moves(player)
    clear_screen()
    print("Welcome to the dungeon!")
    print("You are currently in room {}".format(player))  # fill with player position
    print("you can move {}".format(", ".join(valid_moves)))  # fill with available moves.
    print("Enter QUIT to quit")

    move = input("> ")
    move = move.upper()

    if move == 'QUIT':
        break
    if move in valid_moves:
        player = move_player(player, move)
    else:
        print("\n**Walls are hard!, do not run into them! **\n")
        continue

    # good move?, change the player position.
    # bad move?, do not change anything.
    # on the door? they win!
    # on the monster? they lose!
    # otherwise, loop back around.