Well, to help you solve this problem I will suggest some steps we can take to diagnose:
For the webhook to work in test mode, you have to click the “Execute node” in the node editing window or “Execute workflow” in the main editor window
Once you click the Execute [workflow or node] button, then you are able to access the test URL
You can check if you are trying to access the correct URL by checking the webhook URLs in the top of the node editing window (it is a bit hidden) as shown below:
Let me know if you tried these steps as the error message actually indicates that the webhook is not yet registered, which means probably n8n was not in “test mode” (it goes into this mode when you click the Execute buttons).