Webhook Tester working status

Trying to test my webhook at https://n8n.seeedstack.com/webhook/course/n8n102/new-order with the Webhook Tester - n8n Academy | N8N102

I keep getting this error,

Validation Failed

Please correct the following errors:

  • webhook_url is not valid. Please ensure your hostname resolves and is publicly accessible. Status Code: 500

Please help me out!