In the users_controller.rb -->{I intentionally commented out line 56 because as that reduced the error. I added Line 57 as a work around.}

routes.rb

Rails.application.routes.draw do
  #resources :users
  root 'welcome#home'
  get 'about', to: 'welcome#about'

  resources :articles

  get 'signup', to: 'users#new'
  resources :users, except: [:new]

  get 'login', to: 'sessions#new'
  post 'login', to: 'sessions#create'
  delete 'logout', to: 'sessions#destroy'

  # The priority is based upon order of creation: first created -> highest priority.
  # See how all your routes lay out with "rake routes".

  # You can have the root of your site routed with "root"
  # root 'welcome#index'


  # Example of regular route:
  #   get 'products/:id' => 'catalog#view'

  # Example of named route that can be invoked with purchase_url(id: product.id)
  #   get 'products/:id/purchase' => 'catalog#purchase', as: :purchase

  # Example resource route (maps HTTP verbs to controller actions automatically):
  #   resources :products

  # Example resource route with options:
  #   resources :products do
  #     member do
  #       get 'short'
  #       post 'toggle'
  #     end
  #
  #     collection do
  #       get 'sold'
  #     end
  #   end

  # Example resource route with sub-resources:
  #   resources :products do
  #     resources :comments, :sales
  #     resource :seller
  #   end

  # Example resource route with more complex sub-resources:
  #   resources :products do
  #     resources :comments
  #     resources :sales do
  #       get 'recent', on: :collection
  #     end
  #   end

  # Example resource route with concerns:
  #   concern :toggleable do
  #     post 'toggle'
  #   end
  #   resources :posts, concerns: :toggleable
  #   resources :photos, concerns: :toggleable

  # Example resource route within a namespace:
  #   namespace :admin do
  #     # Directs /admin/products/* to Admin::ProductsController
  #     # (app/controllers/admin/products_controller.rb)
  #     resources :products
  #   end
end

users_controller.rb

class UsersController < ApplicationController
  before_action :set_user, only: [:edit, :update, :show]
  before_action :require_same_user, only: [:edit, :update, :destroy]
  before_action :require_admin, only: [:destroy]
  def index
    @users = User.paginate(page: params[:page], per_page: 5)
  end

  def new
    @user = User.new
  end

  def create
    @user = User.new(user_params)
    if @user.save!
      session[:user_id] = @user.id
      flash[:success] = "Welcome to the alpha blog #{@user.username}"
      redirect_to user_path(@user)
    else
      render 'new'
    end
  end

  def edit
  end

  def update
    if @user.update(user_params)
      flash[:success] = "Your account was updated successfully"
      redirect_to articles_path
    else
      render 'edit'
    end
  end

  def show
    @user_articles = @user.articles.paginate(page: params[:page], per_page: 5)
  end

  def destroy
    @user = User.find(params[:id])
    @user.destroy
    flash[:danger] = "User and all articles created by user have been deleted"
    redirect_to users_path
  end

  private
  def user_params
    params.require(:user).permit(:username, :email, :password)
  end
  def set_user
    @user = User.find(params[:id])
  end

  def require_same_user
    #if current_user != @user and !current_user.admin?
    if current_user.try(:admin?)
      flash[:danger] = "You can only edit your own account"
      redirect_to root_path
    end
  end

  #def require_same_user
    #if current_user.try(:admin?)
      #flash[:danger] = "You can only edit your own account"
      #redirect_to root_path
    #end
  #end

  def require_admin
    if logged_in? and !current_user.admin?
      flash[:danger] = "Only admin users can perform that action"
      redirect_to root_path
    end
  end

end

users_controller_test.rb

require 'test_helper'

class UsersControllerTest < ActionController::TestCase
  setup do
    @user = users(:one)
  end

  test "should get index" do
    get :index
    assert_response :success
    assert_not_nil assigns(:users)
  end

  test "should get new" do
    get :new
    assert_response :success
  end

  test "should create user" do
    assert_difference('User.count') do
      post :create, user: { email: "sample_#{rand(100)}@outlook.com", username: "sample_#{rand(100)}", password: ""}
    end

    assert_redirected_to user_path(assigns(:user))
  end

  test "should show user" do
    get :show, id: @user
    assert_response :success
  end

  test "should get edit" do
    get :edit, id: @user
    assert_response :success
  end

  test "should update user" do
    patch :update, id: @user, user: { email: @user.email, username: @user.username, password: @user.password }
    assert_redirected_to user_path(assigns(:user))
  end

  test "should destroy user" do
    assert_difference('User.count', -1) do
      delete :destroy, id: @user
    end

    assert_redirected_to users_path
  end
end