getting error 415 Unsupported Media Type when sending SMS with sandbox app

Im getting error 415 Unsupported Media Type when sending SMS with sandbox app

APP ID VRbX8zVSRVeOvf0SoKM_1Q

I know my credentials are correct because I can 'GET' the messages I sent using the "try it out"
this is the body of the Json request

{
  "to": [{"phoneNumber": "+15555552586"}],
  "from": {"phoneNumber": "+15555552434"},
  "text": "The ARG is blank"
}

I am using the correct number and extension.
here is the header of the last request

{
  Server: nginx
  Date: Mon, 17 Jan 2022 02:21:20 GMT
  Connection: keep-alive
  RCRequestId: 2865d8f2-773c-11ec-84a0-005056bb9f38
  RoutingKey: SJC12P01PAS02
  X-Rate-Limit-Group: medium
  X-Rate-Limit-Limit: 40
  X-Rate-Limit-Remaining: 39
  X-Rate-Limit-Window: 60
  Content-Type: application/json; charset=utf-8
  Content-Length: 60
  Content-Language: en-US
}

thanks for any and all help