"errorCode": "MSG-242", "message": "The requested feature is not available" on my sandbox account please add feature

I am able to send SMS using sandbox number "+12679304013”.
 When I try to send it shows
{
    "errorCode": "FeatureNotAvailable",
    "message": "The requested feature is not available",
    "errors": [
        {
            "errorCode": "MSG-242",
            "message": "The requested feature is not available"
        }
    ]
}

reference