JSON filter (no order), test at http://jsonpath.com/

data.json

{
  "traceId": "491a1826-0fa1-46d8-970d-704027a2548b",
  "code": "validation.error",
  "message": "Validation failed",
  "errors": [
    {
      "attributeCode": "firstName",
      "messageCode": "validation.empty"
    },
    {
      "attributeCode": "email",
      "messageCode": "validation.empty"
    },
    {
      "attributeCode": "phoneNumber",
      "messageCode": "validation.null"
    },
    {
      "attributeCode": "lastName",
      "messageCode": "validation.empty"
    }
  ]
}

filter

$.errors[?(@.attributeCode=='firstName')].messageCode[0]