I see that you are trying to POST to the Webhook at localhost:5678 for a locally running n8n instance, Besides the recommendation above if you can share the workflow then we can recreate it and check it for you,
I think, if the only difference was the method, the error would be different
(it would say “This webhook is not registered for POST requests. Did you mean to make a GET request?”)
Right now the error says that the “webhook is not registered”. That means the server is not expecting any request to that endpoint, neither GET nor POST.
The error on the screenshot can only mean that the webhook is down.