$_Session variable lost between php file

Question

I got this error when trying to do the forgot password function and got this error: Undefined variable $_Session I start the session and in the config file but somehow the session still lost after I enter the verification code send to the email.

Here is the code of my file. If you need more detail I will add it. Thank you in advance.

// Forget Password
<?php 
session_start();
ob_start();
?>
<div class="right">
    <form action="" method="post" id="formDemo" class="form">
        <div class="form-login">
            <table>
                <tr>
                    <?php
                    require_once 'classes/Users.php';
                    if (isset($_POST['submit'])) {
                        $error = array();
                        $email = $_POST['email'];
                        if ($email == '') {
                            $error['email'] = 'Không được để trống';
                        }
                        if (empty($error)) {
                            $result = $users->getUserEmail($email);
                            $code = substr(rand(0, 999999), 0, 6);
                            $title = 'Quên mật khẩu';
                            $content = "Mã xác nhận của bạn là: <span style='color:green'>" . $code . "</span>";
                            $mail->sendMail($title, $content, $email);

                            $_SESSION['email'] = $email;
                            $_SESSION['code'] = $code;
                            echo '<script>location.href = "./?p=verification";</script>';
                        }
                    }
                    ?>
                </tr>
                <tr>
                    <p style="margin-bottom: 20px" class="title">Forget Password</p>
                </tr>
                <tr>
                    <input type="email" class="control-login" name="email" placeholder="Enter email">
                    <span style="color: red"><?php if (isset($error['email'])) echo $error['email'] ?></span><br><br>
                </tr>

                <tr><br>
                    <div class="login">
                        <button class="login btn btn-light" type="submit" name="submit">Submit</button>
                    </div>
                </tr>

            </table>
        </div>
    </form>
</div>

// Verification
<div class="right">
    <form action="verification.php" method="post" id="formDemo" class="form">
        <div class="form-login">
            <table>

                <tr>
                    <p style="margin-bottom: 20px" class="title">Enter verification code</p>
                </tr>
                <tr>
                    <?php if (isset($_POST['submit'])) {
                        $error = array();
                        if ($_POST['text'] != $_SESSION['code']) {
                            $error['fail'] = 'Mã xác nhận không hợp lệ';
                        } else {
                            echo '<script>location.href = "./?p=resetPass";</script>';
                        }
                    }
                    ?>
                </tr>
                <tr>
                    <?php if (isset($error['fail'])): ?>
                        <div class="alert alert-primary" role="alert">
                            <?= $error['fail'] ?>
                        </div>
                    <?php else : ?>
                        <div class="alert alert-primary" role="alert">
                            Please enter the verification code we send to your email
                        </div>
                    <?php endif ?>

                </tr>
                <tr>
                    <input type="text" class="control-login" name="text" placeholder="Verification code">
                </tr>
                <tr><br><br>
                    <div class="login">
                        <button class="login btn btn-light" type="submit" name="submit">Send</button>
                    </div>
                </tr>
            </table>
        </div>
    </form>
</div>