Error 2015: Mentioned Handle left the Chat

{
  "error": {
    "status": 409,
    "code": 2015,
    "message": "The structured mention targets a Contact that left or was removed.",
    "doc_url": "https://docs.relayapp.im/error/codes/2xxx/2015"
  },
  "success": false,
  "trace_id": "b923d71e50be43ba9e0fe1e34a7676c2"
}