HA requirements -HA interface must be UP and Peer Device must be reachable

https://docs.sophos.com/nsg/sophos-firewall/19.0/Help/en-us/webhelp/onlinehelp/HighAvailablityStartupGuide/AboutHA/HAPrerequisitesStartupGuide/index.html#networking-and-access-policy

Please add the following point:

• HA interface must be UP and Peer Device must be reachable (otherwise you will get the non-descriptive Error "HA could not be enabled" while Setup Primary Device in HA Interactive Mode (active-passiv) without Link to the Peer Device. Cost me some hours troubleshooting this if Firewalls are in different Datacenters)