Undefined Method attr_accessible

No mass assignment allowed for Rails 4.1

instead of having attr_accessible :username, :email, :password, :password_confirmation in your model, use strong parameters.
You’ll do this in your UsersController:

    def user_params
      params.require(:user).permit(:username, :email, :password, :password_confirmation)
    end

then call the user_params method in your controller actions.