Database error: failed to registerが、解決できない

前提

ログインシステムを作成しています。
ユーザー情報をテーブルにINSERTする工程で問題が発生しています。

実現したいこと

usersテーブルにユーザー情報を格納したい
エラーを解消したい

発生している問題・エラーメッセージ

// signup.php
Database error: failed to register

該当のソースコード

PHP
// authController.php

<?php
ini_set('display_errors', 1);

session_start();
require '../config/db.php';

$errors = array();
$username = '';
$email = '';

if (isset($_POST['signup-btn'])) {
    $username = $_POST['username'];
    $email = $_POST['email'];
    $password_1 = $_POST['password_1'];
    $password_2 = $_POST['password_2'];

    if (empty($username)) {
        $errors['username'] = "Username required";
    }
    if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
        $errors['email'] = "Email address invalid";
    }
    if (empty($email)) {
        $errors['email'] = "Email required";
    }
    if (empty($password_1)) {
        $errors['password'] = "Password required";
    }
    if ($password_1 !== $password_2) {
        $errors['password'] = "The two password do not match";
    }

    $emailQuery = "SELECT * FROM users WHERE email = ? LIMIT 1";
    $statement = $conn->prepare($emailQuery);
    $statement->bind_param('s', $email);
    $statement->execute();
    $result = $statement->get_result();
    $userCount = $result->num_rows;

    $statement->close();

    if ($userCount > 0) {
        $errors['email'] = "Email already exists";
    }

    if (count($errors) === 0) {
        $password = password_hash($password_1, PASSWORD_DEFAULT);
        $token = bin2hex(random_bytes(50));
        $verified = false;

        $sql = "INSERT INTO users (username, email, verified, token, password) VALUES (?, ?, ?, ?, ?)";
        $statement = $conn->prepare($sql);
        $statement->bind_param('ssbss', $username, $email, $verified, $token, $password);
        if ($statement->execute()) {
            $user_id = $conn->insert_id;
            $_SESSION['id'] = $user_id;
            $_SESSION['username'] = $username;
            $_SESSION['email'] = $email;
            $_SESSION['verified'] = $verified;

            $_SESSION['message'] = "You are now logged in!";
            $_SESSION['alert-class'] = "alert-success";
            header('location: index.php');
            exit();
        } else {
            $errors['db_error'] = "Database error: failed to register";
        }
    }
}

PHP
// signup.php

<?php
require_once '../controllers/authController.php';

?>

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Register</title>
</head>

<body>
    <div>
        <div>
            <form action="signup.php" method="POST">
                <h3>Register</h3>
                <?php if (count($errors) > 0) : ?>
                    <div>
                        <?php foreach ($errors as $error) : ?>
                            <li><?php echo $error; ?></li>
                        <?php endforeach; ?>
                    </div>
                <?php endif; ?>
                <div>
                    <label for="username">Username</label>
                    <input type="text" name="username" value="<?php echo $username; ?>">
                </div>
                <div>
                    <label for="email">Email</label>
                    <input type="email" name="email" value="<?php echo $email; ?>">
                </div>
                <div>
                    <label for="password_1">Password</label>
                    <input type="password" name="password_1">
                </div>
                <div>
                    <label for="password_2">Confirm Password</label>
                    <input type="password" name="password_2">
                </div>
                <div>
                    <button type="submit" name="signup-btn">Sign Up</button>
                </div>
                <p>Already a member? <a href="login.php">Sign In</a></p>
            </form>
        </div>
    </div>
</body>

</html>

PHP
// index.php

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Homepage</title>
</head>

<body>
    <div>
        <div>
            <div>
                You are now logged in!
            </div>

            <h3>Welcome, Awa</h3>

            <a href="#">logout</a>

            <div>
                You need to verify your account.
                Sign in to your email account and click on the
                verification link we just emailed you at
                <strong>test@test.com</strong>
            </div>
        </div>

        <button>I am verified!</button>
    </div>
</body>

</html>

PHP
// db.php

<?php
require 'constants.php';

$conn = new mysqli(DB_HOST, DB_USER, DB_PASS, DB_NAME);

if ($conn->connect_error) {
    die("Database error:" . $conn->connect_error);
}

MySQL
mysql> show columns from users;
+----------+--------------+------+-----+---------+-------+
| Field    | Type         | Null | Key | Default | Extra |
+----------+--------------+------+-----+---------+-------+
| id       | int          | NO   | PRI | NULL    |       |
| username | varchar(100) | YES  |     | NULL    |       |
| email    | varchar(200) | YES  | UNI | NULL    |       |
| verified | tinyint      | YES  |     | NULL    |       |
| token    | varchar(100) | YES  |     | NULL    |       |
| password | varchar(255) | YES  |     | NULL    |       |
+----------+--------------+------+-----+---------+-------+