I'm creating Space Invader game and I'm unable to fire bullets. I'm following a tutorial by freecodecamp on youtube

Question

here, I'm following every instructions step by step, all files are in same folder, even the code is almost same, still facing this issue. Please help me out! also, please explain me the cause of the error as I'm a complete beginner at pygames and stackoverflow (posting my 1st question!)

import pygame
import random

pygame.init()

Game Screen, Icon and background

screen = pygame.display.set_mode((800, 600))
background = pygame.image.load('background.png')
pygame.display.set_caption("Space Invaders")
icon = pygame.image.load('ufo.png')
pygame.display.set_icon(icon)

Player

playerImg = pygame.image.load('player.png')
playerX = 370
playerY = 480
playerX_change = 0

Enemy

enemyImg = pygame.image.load('enemy.png')
enemyX = random.randint(0, 800)
enemyY = random.randint(50, 150)
enemyX_change = 4
enemyY_change = 40

Bullet (Probably error)

bulletImg = pygame.image.load('bullet.png')
bulletX = 0
bulletY = 480
bulletX_change = 0
bulletY_change = 10
bullet_state = "ready"

Functions

def player(x, y):
    screen.blit(playerImg, (x, y))


def enemy(x, y):
    screen.blit(enemyImg, (x, y))


def fire_bullet(x, y):
    global bullet_state
    bullet_state = "fire"
    screen.blit(bulletImg, (x + 16, y + 10))

Game running Loop

running = True
while running:

    screen.fill((0, 0, 0))

    screen.blit(background, (0, 0))

    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False

        if event.type == pygame.KEYDOWN:
            if event.key == pygame.K_LEFT:
                playerX_change = -5
            if event.key == pygame.K_RIGHT:
                playerX_change = 5
            if event.type == pygame.K_SPACE:
                fire_bullet(playerX, bulletY)

        if event.type == pygame.KEYUP:
            if event.key == pygame.K_LEFT or event.key == pygame.K_RIGHT:
                playerX_change = 0

    playerX += playerX_change

    if playerX <= 0:
        playerX = 0
    elif playerX >= 736:
        playerX = 736

    enemyX += enemyX_change

    if enemyX <= 0:
        enemyX_change = 4
        enemyY += enemyY_change
    elif enemyX >= 736:
        enemyX_change = -4
        enemyY += enemyY_change

    if bullet_state == "fire":
        fire_bullet(playerX, bulletY)
        bulletY -= bulletY_change

    player(playerX, playerY)
    enemy(enemyX, enemyY)
    pygame.display.update()