To register with a hub your Moodle site needs to be accessible by the hub so the two can communicate with one another.
The error indicates that the hub could not reach your site. Most often this is because your site is not online, but only exists on your computer.
Reasons why you may have seen this error:
- Your site has a local address like http://localhost or http://127.0.0.1 - in other words, your site is not on the internet so Moodle cannot see it.
- Your site is protected by a proxy or firewall
- The hub is being prevented in some other way from accessing your site - for example you have restrictions on certain IP addresses, one of which might be the hub.