This message happens because you are requesting the sigin page directly while it is supposed that this page comes up as a challenge when you request s resource protected by IdCS in wich you are not logged in yet.
Solution: Request a resource protected by IdCS such as an application or this url <youridentitydomainurl>/ui/v1/myconsole. If you don’t have an application already registered and have administrative privileges try with <youridentitydomainurl>/ui/v1/ adminconsole
Enjoy 😉
approaching off the bow 