[Answered ]-'CategoryDetailView' object has no attribute 'get_object'

You are working with a ListView [Django-doc], not a DetailView [Django-doc], and a ListView indeed has no .get_object(…) method [Django-doc]. You furthermore should specify a model = … or queryset = … to specify with what queryset the DetailView is dealing.

You thus should inherit from the DetailView. You can also work with self.object to prevent making an extra query:

from django.views.generic import DetailView

class CategoryDetailView(LoginRequiredMixin, DetailView):
    template_name = 'clients/category/category_detail.html'
    context_object_name = 'category'
    model = Category

    def get_context_data(self, **kwargs):
        context = super().get_context_data(**kwargs)
        clients = self.object.client_set.all()
        context['clients'] = clients
        return context

There is als no reason to add this to the context, in the template, you can simply work with:

{% for client in category.client_set.all %}
    {{ client }}
{% endfor %}