So I have installed license and shield plugins, after which I get user authentication error. I guess it defaults to Shield realm. What's the default user/password ? Also, how do I manually create users from a command line?
I don't think there's a default user. You have to create them first, from command line.
Here: https://www.elastic.co/guide/en/shield/current/enable-basic-auth.html
There is no default for 2.X, for 5.X it's elastic and password.