How to return custom error messages in django api view

I am trying to check if the status of a buyer is equal to buyer_completed, then i perform some operations, i also want to return a response if the conditions does not match, but the error seems not to be showing as expected.

class SellerCompleteOrder(generics.RetrieveUpdateAPIView):
    serializer_class = OrderSerializer
    # permission_classes = [IsAuthenticated]
    def get_object(self):
        user_id = self.kwargs['user_id']
        order_id = self.kwargs['order_id']

        user = User.objects.get(id=user_id)
        order = Orders.objects.get(seller=user, id=order_id)
        if order.buyer_status == "buyer_completed":
            order.seller_status = "seller_completed"
            return Response({"Seller Have Marked Order as Completed":order})
            return Response({"Seller cannot mark order as completed":order})

this is the response

The get_object function should not return a Response object, but the a model instance.
You maybe want to override the list or retrieve method, both of them are defined on the generics.RetrieveUpdateAPIView