The user interface is a portal that allows the loged-in user to see all the applications he is allowed to use, and access them. This interface is also where the user can set his parameters (change password, define email alias and redirections).
Access to user interface can only be done by using a domain name. You can’t access to it by using the local IP address of your server like that: https://your.server.ip.address/yunohost/sso/
You can create a redirection of a domain name to IP address of your server by modifing
/etc/hosts and by adding that line:
your.server.ip.address for instance by
domain.tld by your domain name or by one you can choose if you doesn’t own one.
User interface is based on that software: https://github.com/Kloadut/ssowat