<myObject> object is not iterable, Django REST Framework

I'm getting the following error when posting a new Product to my application

Traceback (most recent call last):
  File "/mnt/c/Users/connect/Desktop/PIM/env/lib/python3.6/site-packages/django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "/mnt/c/Users/connect/Desktop/PIM/env/lib/python3.6/site-packages/django/core/handlers/base.py", line 126, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/mnt/c/Users/connect/Desktop/PIM/env/lib/python3.6/site-packages/django/core/handlers/base.py", line 124, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/mnt/c/Users/connect/Desktop/PIM/env/lib/python3.6/site-packages/django/views/decorators/csrf.py", line 54, in wrapped_view
    return view_func(*args, **kwargs)
  File "/mnt/c/Users/connect/Desktop/PIM/env/lib/python3.6/site-packages/rest_framework/viewsets.py", line 116, in view
    return self.dispatch(request, *args, **kwargs)
  File "/mnt/c/Users/connect/Desktop/PIM/env/lib/python3.6/site-packages/rest_framework/views.py", line 495, in dispatch
    response = self.handle_exception(exc)
  File "/mnt/c/Users/connect/Desktop/PIM/env/lib/python3.6/site-packages/rest_framework/views.py", line 455, in handle_exception
    self.raise_uncaught_exception(exc)
  File "/mnt/c/Users/connect/Desktop/PIM/env/lib/python3.6/site-packages/rest_framework/views.py", line 492, in dispatch
    response = handler(request, *args, **kwargs)
  File "/mnt/c/Users/connect/Desktop/PIM/env/lib/python3.6/site-packages/rest_framework/mixins.py", line 20, in create
    serializer.is_valid(raise_exception=True)
  File "/mnt/c/Users/connect/Desktop/PIM/env/lib/python3.6/site-packages/rest_framework/serializers.py", line 236, in is_valid
    self._validated_data = self.run_validation(self.initial_data)
  File "/mnt/c/Users/connect/Desktop/PIM/env/lib/python3.6/site-packages/rest_framework/serializers.py", line 434, in run_validation
    value = self.to_internal_value(data)
  File "/mnt/c/Users/connect/Desktop/PIM/env/lib/python3.6/site-packages/rest_framework/serializers.py", line 488, in to_internal_value
    validated_value = field.run_validation(primitive_value)
  File "/mnt/c/Users/connect/Desktop/PIM/env/lib/python3.6/site-packages/rest_framework/serializers.py", line 623, in run_validation
    value = self.to_internal_value(data)
  File "/mnt/c/Users/connect/Desktop/PIM/env/lib/python3.6/site-packages/rest_framework/serializers.py", line 662, in to_internal_value
    validated = self.child.run_validation(item)
  File "/mnt/c/Users/connect/Desktop/PIM/env/lib/python3.6/site-packages/rest_framework/serializers.py", line 436, in run_validation
    self.run_validators(value)
  File "/mnt/c/Users/connect/Desktop/PIM/env/lib/python3.6/site-packages/rest_framework/serializers.py", line 465, in run_validators
    to_validate.update(value)
TypeError: 'Category' object is not iterable

my serializer.py

from rest_framework import serializers
from products_and_categories.models import Product, Category
from django.db import models

class CategorySerializer(serializers.ModelSerializer):
    class Meta:
        model = Category
        fields = ('id', "name", 'products', 'categories')

    def to_representation(self, obj):
        if 'categories' not in self.fields:
            self.fields['categories'] = CategorySerializer(obj, many=True)      
        return super(CategorySerializer, self).to_representation(obj)

    def to_internal_value(self, data):
        try:
            try:
                obj_id = data['id']
                return Category.objects.get(id=obj_id)
            except KeyError:
                raise serializers.ValidationError(
                    'id is a required field.'
                )
            except ValueError:
                raise serializers.ValidationError(
                    'id must be an integer.'
                )
        except Category.DoesNotExist:
            raise serializers.ValidationError(
            'Obj does not exist.'
            )


class ProductSerializer(serializers.ModelSerializer):
    categories = CategorySerializer(many=True)#, queryset= Category.objects.all(), slug_field='categories')

    class Meta:
        model = Product
        fields = ('id', "product_code", "name", "quantity", "price", 'categories')
        #.....

Any idea why this is happening ?

I don't quiet understand what it means by it's not iterable, what part of the code exactly is causing this error?

I would really appreciate any help

the post is in the following format

{

"id": 22,

"product_code": "dklj",

"name": "kdlj",

"quantity": 15,
"price": 15,
"categories": [ {
    "id": 5,
    "name": "Test category 3",
    "products": [],
    "categories": []
}]

}