Routing Error uninitialized constant Adminの対処方法について

Question

###前提・実現したいこと
Ruby on rails にてサービスのログイン機能機能を作成しております。
こちらのサイトに倣って作成しておりましたが、最後の「Articles」に「[e-mail]」でログインする際に、下記のエラーメッセージが表示されてしまいます。
この対処方法を教えて頂けないでしょうか？
※その他不足情報ございましたら、ご教示願います。

###エラーメッセージ
Routing Error
uninitialized constant Admin

Rails.root: /Users/TOMOAKI/chinaEC

Application Trace | Framework Trace | Full Trace
Routes

Routes match in priority from top to bottom

Helper HTTP Verb Path Controller#Action
Path / Url
articles_path GET /articles(.:format) articles#index
POST /articles(.:format) articles#create
new_article_path GET /articles/new(.:format) articles#new
edit_article_path GET /articles/:id/edit(.:format) articles#edit
article_path GET /articles/:id(.:format) articles#show
PATCH /articles/:id(.:format) articles#update
PUT /articles/:id(.:format) articles#update
DELETE /articles/:id(.:format) articles#destroy
rails_admin_path /admin RailsAdmin::Engine
new_user_session_path GET /users/sign_in(.:format) devise/sessions#new
user_session_path POST /users/sign_in(.:format) devise/sessions#create
destroy_user_session_path GET /users/sign_out(.:format) devise/sessions#destroy
user_password_path POST /users/password(.:format) devise/passwords#create
new_user_password_path GET /users/password/new(.:format) devise/passwords#new
edit_user_password_path GET /users/password/edit(.:format) devise/passwords#edit
PATCH /users/password(.:format) devise/passwords#update
PUT /users/password(.:format) devise/passwords#update
cancel_user_registration_path GET /users/cancel(.:format) devise/registrations#cancel
user_registration_path POST /users(.:format) devise/registrations#create
new_user_registration_path GET /users/sign_up(.:format) devise/registrations#new
edit_user_registration_path GET /users/edit(.:format) devise/registrations#edit
PATCH /users(.:format) devise/registrations#update
PUT /users(.:format) devise/registrations#update
DELETE /users(.:format) devise/registrations#destroy
root_path GET / admin/base#index
admin_path GET /admin(.:format) admin/base#index
Routes for RailsAdmin::Engine
dashboard_path GET / rails_admin/main#dashboard
index_path GET|POST /:model_name(.:format) rails_admin/main#index
new_path GET|POST /:model_name/new(.:format) rails_admin/main#new
export_path GET|POST /:model_name/export(.:format) rails_admin/main#export
bulk_delete_path POST|DELETE /:model_name/bulk_delete(.:format) rails_admin/main#bulk_delete
bulk_action_path POST /:model_name/bulk_action(.:format) rails_admin/main#bulk_action
show_path GET /:model_name/:id(.:format) rails_admin/main#show
edit_path GET|PUT /:model_name/:id/edit(.:format) rails_admin/main#edit
delete_path GET|DELETE /:model_name/:id/delete(.:format) rails_admin/main#delete
show_in_app_path GET /:model_name/:id/show_in_app(.:format) rails_admin/main#show_in_app
Request

Parameters:

None
Toggle session dump
Toggle env dump
Response

Headers:

None

###Config>locales>routes.rb

Rails.application.routes.draw do
  resources :articles
  mount RailsAdmin::Engine => '/admin', as: 'rails_admin'
  devise_for :users

  root 'admin/base#index' # ログイン画面をルートにする
  get '/admin' => 'admin/base#index'
  # 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

##models>concerns>ability.rb

class Ability
  include CanCan::Ability

  def initialize(user)
    # Define abilities for the passed in user here. For example:
    #
    #   user ||= User.new # guest user (not logged in)
    #   if user.admin?
    #     can :manage, :all
    #   else
    #     can :read, :all
    #   end
    #
    # The first argument to `can` is the action you are giving the user
    # permission to do.
    # If you pass :manage it will apply to every action. Other common actions
    # here are :read, :create, :update and :destroy.
    #
    # The second argument is the resource the user can perform the action on.
    # If you pass :all it will apply to every resource. Otherwise pass a Ruby
    # class of the resource.
    #
    # The third argument is an optional hash of conditions to further filter the
    # objects.
    # For example, here the user can only update published articles.
    #
    #   can :update, Article, :published => true
    #
    # See the wiki for details:
    # https://github.com/CanCanCommunity/cancancan/wiki/Defining-Abilities
    user ||= User.new # guest user (not logged in)
    if user.has_role?('admin')
      can :manage, :all
    else
      can :manage, :articles
    end  
  end
end

Answer 1

https://teratail.com/questions/29697
これと同じエラーですね。