Hi Rajesh,
Create_entity or create_deep_entity will be triggered based on the payload. If you are passing a payload with nested entity , call will go to create_deep_entity.
This error comes when the request payload is not corrext. First I would suggest you to put a breakpoint in the create_entity/create_deep_entity method and to check if the call is reaching these methods.
Then to create the correct payload, fire a READ or READ with $EXPAND (based on the requirement) with $format=json. Then use the response as the payload for POST. Ensure you are passing content type = application/json in the header.
Regards,
Atanu