Apps Script: You do not have permission to call UrlFetchApp.fetch

The running script does not hold the script.external_request OAuth scope, which UrlFetchApp needs to contact external URLs. The message names the missing scope directly.

There are two common situations. First, the function runs in a context that cannot use authorized services at all: Google documents that simple triggers such as onEdit and onOpen can't access services that require authorization. Second, the project's appsscript.json sets oauthScopes explicitly; Google's reference says you must then add this scope manually, because automatic scope detection is turned off.

The same pattern produces You do not have permission to call messages for other services, such as MailApp.sendEmail or SpreadsheetApp.openById.

// appsscript.json
{
  "oauthScopes": [
    "https://www.googleapis.com/auth/spreadsheets.currentonly",
    "https://www.googleapis.com/auth/script.external_request"
  ]
}

Keep simple triggers for lightweight, local work only; anything that sends mail, opens other files or calls the network belongs in an installable trigger. If you declare scopes manually, review the list whenever you add a new service call.