how can i return error message on login form (which is in bootstrap modal)

You can use a Http404 condition to render error message on login. Import Http404 the class is django.http.Http404 if you raise any error in that case Django will return the error message or page that you set to be displayed ex: 404.html. You code may look like this:

def login_view(request):
    print(request.user.is_authenticated())
    next = request.GET.get('next')
    title = "Login"
    form = UserLoginForm(request.POST or None)
    try:
        if form.is_valid():    
            username = form.cleaned_data.get("username")
            password = form.cleaned_data.get('password')
            user = authenticate(username=username, password=password)
            login(request, user)        
            if next:
                return redirect(next)
            return redirect("/userdashboard")
    except:
        raise Http404 (" This user does not exist.")
    return render(request, "index.html", {"form":form, "title": title})

inside the bootstrap modal use form errors in loop.

{% if form.errors %}
    {% for field in form %}
        {% for error in field.errors %}
            <div class="alert alert-danger">
                <strong>{{ error|escape }}</strong>
            </div>
        {% endfor %}
    {% endfor %}
    {% for error in form.non_field_errors %}
        <div class="alert alert-danger">
            <strong>{{ error|escape }}</strong>
        </div>
    {% endfor %}
{% endif %}