As others have replied here, it is not possible to login as the system user. The system user is a special account that is automatically created by Discourse. It is used for performing some automated functions on the site. It is also used as the API user if you make API requests with the All Users API key.
If the site that you are asking about is on our hosting, you do not have access to the Rails console, so the guide for creating accounts from the Rails console will not apply to you.
Have a look at this topic What is a staged user?
If you can give us specific details about what you are trying to accomplish, we will try to help.