注册表单返回服务器错误500

我创建了一个表单，用户在其中输入他们的注册详细信息，并使用查询将其输入到SQL数据库中。但是，我输入了所有详细信息，单击submit，结果返回服务器错误500。我是装订新手。执行这个命令可能是由于mysqli行格式的问题。

registrationsubmit.php

<font face="ClearSans-Thin">
<font color="lightgray">
<?php
include 'registrationform.php';
include 'connection.php';
?>
<center>
<?php
if (isset($_POST['regsubmit'])) {
    $firstname = $_POST['firstname'];
    $lastname = $_POST['lastname'];
    $user = $_POST['username'];    
    $pass = $_POST['password'];
    if ($stmt = mysqli_prepare($dbhandle, $query = "INSERT INTO users (FirstName, LastName, Username, Password) VALUES (?,?,?,?)")) {
    mysqli_stmt_bind_param($stmt, 'ssss', $firstname,$lastname,$user,$pass);
    mysqli_stmt_execute($stmt) or die('This username is already in use - Please choose another one');
 }else{
die('Error when preparing '.mysqli_error($dbhandle));
}
  Echo "Registration Successful - Press the Home button to return to the homepage.";
}
?>
</center>