Couldn't reach that endpoint URL.
The reachability check against your configured endpoint URL didn't get a response - this isn't Meta's own health check, just confirming something answers at that address at all.
Check that your endpoint server is up and running.
Check that the endpoint URL is correct.
Try again.
Still stuck? Send AVX-3048 to hello@theavoxai.com along with what you were doing when it happened.