I have a new installation of 10.6 on machine A in it's own site.
I installed 10.6 on machine B.
When I try to join machine B to machine A's Site, I get a message stating:
Failed to join the site 'https://xxx.xxx.xxx.xxx:6443/arcgis/admin/' Enable SSL Failed.
I have confirmed on machine A, via admin/security/config that HTTP And HTTPS Protocols are enabled.
I get the same Enable SSL Failed message if I try to add machine B from machine A via Manager.