Error 1004: Handle is not in the Chat

{
  "error": {
    "status": 400,
    "code": 1004,
    "message": "The mentioned Handle is not an active Contact in this Chat.",
    "doc_url": "https://docs.relayapp.im/error/codes/1xxx/1004"
  },
  "success": false,
  "trace_id": "b923d71e50be43ba9e0fe1e34a7676c2"
}