I had the same issue with 2018.1. I submitted a ticket and this is the response I received from support (note, I reset IIS on the server after following the instructions below). Hope it helps.
I would request you to login to Orchestrator with the below credentials, this is the admin user to manage tenant.
Below is the default credentials:
Tenant Name: Host
User: admin
pwd: 3edcVFR$
Once you are logged in → Go to Right Hand Side top Corner → Click on the user → Settings → Application → And then “Enable tenant Registration”
Note:- There is some issue with enabling tenant registration from the web.config file. I will escalate this to my dev team and in the meanwhile please use the workaround mentioned above.