Error Unauthorized when trigger a flow with Webhook and HTTP request

May follow below steps to address your issue:

• Check Authentication Headers: Ensure that the webhook request from Azure DevOps includes the necessary authentication headers. This might involve setting up a service principal or using a personal access token (PAT) for authentication.
• Verify Permissions: Make sure that the account or service principal used for the webhook has the necessary permissions to trigger the Flow. This includes permissions in both Azure DevOps and the target service.
• Use a Secure Token: If you're using a token for authentication, ensure that it is correctly included in the webhook URL or headers. Some services require the token to be passed as a query parameter or in the authorization header.
• Check Endpoint Configuration: Verify that the endpoint URL is correctly configured and accessible. Sometimes, minor typos or incorrect URLs can cause authentication issues.
• Review Logs: Check the logs in both Azure DevOps and the target service to identify any specific error messages or codes that can provide more insights into the issue.
• Test with CURL or Postman: Use tools like CURL or Postman to manually send a request to the webhook URL with the same headers and payload. This can help you identify if the issue is with the webhook configuration or the request itself.