The error "System.CalloutException: Unauthorized endpoint" indicates that the external endpoint is not whitelisted in Salesforce. To allow Apex callouts to access an external endpoint, the administrator or the developer must register the URL in Remote Site Settings. This is a security feature that prevents unauthorized access to external resources from Apex code. Creating a connected app for the external system or ensuring that the integration user has the necessary permissions to perform the callout are not sufficient to fix this error, as they do not address the issue of whitelisting the endpoint12. References: Apex Developer Guide: Making HTTP Callouts, Apex Developer Guide: Remote Site Settings