Hi - Have you read through the administration guide regarding setting up users? You need to add some details to contacts and send invites to the site to be able to sign in for the first time. All site users are contacts so you will also need
to add any internal admin users as contacts and invite them also (as well as relating them to the admin role if you want them to be able to content manage the site). When I first deployed the site I has the same situation as you but everything worked
fine after following the admin guide.
Regards
Andrew