Hey @kgurinov
There are a couple of things you have to check.
Can you make sure you’re using Test URL (for Testing Purpose) and POST is selected?
If it’s fine can you check you have WEBHOOK_TUNNEL_URL in the Environment Variables?
It should have your n8n instance URL.
example: https://n8n.mydomain.com
If it’s not set, You can refer to this part of the documentation to set your Webhook Tunnel URL
or Watch this video (Included Timestamp) by @harshil1712
