Почему в форме регистрации HTML появляется сообщение об ошибке «Cannot POST to php»?

Question

Я создаю регистрационную форму и просто хочу вставить данные в свою базу данных.

Это мой php-код, который используется в "action = " части моей формы.

$username = $_POST['username'];
$password = $_POST['password'];

$repeatpassword = $_POST['repeatpassword'];

$age = $_POST['age'];

$sexuality = $_POST['sexuality'];

$gender = $_POST['gender'];

$topic = $_POST['topic'];

if (!empty($username) || !empty($password)) {

    $host = "localhost";
    $dbUsername = "root";
    $dbPassword = "";
    $dbName = "register";

    $conn = new mysqli($host, $dbUsername, $dbPassword, $dbName);

    if (mysqli_connect_error()) {
        die('Connect Error('. mysqli_connect_errno().')'. mysqli_connect_error());
    }
    else {
        $SELECT = "SELECT username From users Where username = ? Limit 1";
        $INSERT = "INSERT Into users (username, password, age, sexuality, gender, topic) values(?, ?, ?, ?, ?, ?)";

    }

    $stmt = $conn->prepare($SELECT);
    $stmt->bind_param("s", $username);
    $stmt->execute();
    $stmt->bind_result($username);
    $stmt->store_result();
    $rnum = $stmt->num_rows;

    if ($rnum == 0) {
      $stmt->close();
      $stmt = $conn->prepare($INSERT);
      $stmt->bind_param("ssisss", $username, $password, $age, $sexuality, $gender, $topic);
      $stmt->execute();

      echo "Thank you for registering!";
    } 
    else {
      echo "Someone already registered using this username!";
    }
    $stmt->close();
    $conn->close();


} 

else {

    echo "Please fill in all fields!";
    die();
}