You need to have a button to submit.

<input type="submit">

Try that :

<h3>Create your account!</h3>
<form method="post" action="/signup">
    {% csrf_token %}
    <div class="form-group">
        <label for="username">Username</label>
        <input type="text" class="form-control" id="username" name="username" placeholder="Create A Username (use only letters and numbers)" Required>
    </div>
    <input type="submit">
   </form>

Your form was not a valid one and was not submitting anything.