Your configuration instructions for High Availability is incomplete.
The HA setup can not proceed until AFTER both firewalls are registered.
I have had to contact support multiple more than once due to this issue.
Add a section on the HA setup page on your website, stating the requirement of registration:
https://docs.sophos.com/nsg/sophos-firewall/19.5/Help/en-us/webhelp/onlinehelp/HighAvailablityStartupGuide/HAConfiguration/HAQuickHAConfigureActivePassive/index.html
https://docs.sophos.com/nsg/sophos-firewall/21.0/help/en-us/webhelp/onlinehelp/HighAvailablityStartupGuide/HAConfiguration/HAQuickHAConfigureActivePassive/index.html
And a statement of registration being required before setup does need to also be added to the HA configuration page inside the firewalls.
But prerequisite check before starting HA setup and useful error messages are the most important change that should have been completed the last time I contacted support about this issue in your firewall firmware.
