Error sending form. This simple code is not working

<form>
<div class="row">
    <div class="col-sm-12 form-group">
        <input class="form-control" id="name" name="name" placeholder="Nome" type="text" required>
    </div>
</div>
<div class="row">
    <div class="col-sm-12 form-group">
        <input class="form-control" id="email" name="email" placeholder="Email" type="email" required>
    </div>
</div>
<div class="row">
    <div class="col-sm-12 form-group">
        <select class="form-control" name="subject" required>
            <option>Assunto</option>
            <option>Orçamento Materiais</option>
            <option>Sugestão</option>
            <option>Trabalhe Conosco</option>
            <option>Outro</option>
        </select>
    </div>
</div>                      
<div class="row">
    <div class="col-sm-12 form-group">
        <textarea class="form-control" id="message" name="message" placeholder="Mensagem" rows="8" required></textarea><br>
    </div>
</div>
<div class="row">
    <div class="col-sm-12 form-group">
        <button type="submit" class="btn btn-primary btn-block btn-lg" name="send" style="border-radius:0px;">ENVIAR</button>
    </div>
</div>
</form>

<?php
//Variáveis enviadas pelo formulário
$nome = $_POST["name"];
$emailUsuario = $_POST["email"];
$assunto = $_POST["subject"];
$sugestao = $_POST["message"];

//Testando
echo $nome;
echo "<br>";
echo $emailUsuario;
echo "<br>";
echo $assunto;
echo "<br>";
echo $sugestao;
echo "<br>";    
?>