To register to a hub your Moodle site needs to be accessible by the hub so the two can communicate bi-directionally.
The error indicates that the hub could not reach your site.
Reasons why you may have seen this error:
- Your site has a local address like http://localhost or http://127.0.0.1
- 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.