Can't sign in as admin user (undefined method `username' for nil:NilClass)

I’m not going to use it on production. I’m trying to install in on my local dev server. The reason is what I want to make some changes to default template. For example I want best authors (most liked) to be shown on front page. Right now I have no idea how to do this. I think it would be easier to understand how template works on dev environment. But that’s another story…