<form method="GET" action="{% url 'register' activity.pk %}">
             {% csrf_token %}
            <button
            type="submit"
            class="btn btn-primary"
            id="Register">Register
            </button>
    </form>

url(r'^(?P<pk>)', authentication_views.registerAttende, name='register'),

def registerAttende(request,pk):
    act = Activity.objects.get(pk=pk) //This is the line that is causing the error
    act.save()
    attend = act.attendee.create(student=request.user.id)
    return render(request, 'dashboard.html')