not_found

The API returns not_found with HTTP status 404.

The resource doesn't exist, or doesn't belong to you.

#Where the API reference mentions it

• Steps (Campaigns)

#What the response looks like

{
  "error": {
    "code": "not_found",
    "message": "...",
    "details": null
  }
}

error.code is stable and safe to branch on. error.message is debugging context, not something to parse. The full list of codes, and the rate limits, are on Errors & Rate Limits.