Not entitled to use a custom response

Developers API
1 
{
    "result": null,
    "success": false,
    "errors": [
        {
            "message": "not entitled to use a custom response",
            "source": {
                "pointer": "/rules/0"
            }
        }
    ],
    "messages": null
}

I got this response which from “http_request_firewall_custom” API. What it does means?