We couldn't reach that webhook URL.
The "Test endpoint" step couldn't get a response from your webhook URL. Check that it's publicly reachable over HTTPS, then try again before continuing to Meta.
Check that your webhook URL is publicly reachable from the internet, not localhost or behind a firewall.
Check that it uses HTTPS.
Run the "Test endpoint" step again.
Only continue to Meta once the test succeeds.
Still stuck? Send AVX-3008 to hello@theavoxai.com along with what you were doing when it happened.