【お助けください】No route matches [GET] "/auth/google_oauth2"を解決したい

Question

■環境
OS: macOS Big Sur(M1 mac) 11.5.2
Ruby: 3.0.0p0 (2020-12-25 revision 95aff21468) [arm64-darwin20]
Rails: 6.1.4.1

■背景
deviseを使用せず、Google認証で新規登録・ログイン画面を作成しようとしているのですが、
routeが通らず、コールバックされない状況です。
OAuthクライアントIDの作成や承認済みURIの設定は完了しているのですが、
何が原因でrouteエラーになっているのでしょうか？

■補足
「deviseなしでGoogleOAuth認証のサインインの実装」を参考にGoogle Developers Consoleの登録とRailsを実装しました。

■エラー内容: development.log

ActionController::RoutingError (No route matches [GET] "/auth/google_oauth2"):

■callbackのURLを設定

■Gemfile

source 'https://rubygems.org'
git_source(:github) { |repo| "https://github.com/#{repo}.git" }

ruby '3.0.0'

# Bundle edge Rails instead: gem 'rails', github: 'rails/rails', branch: 'main'
gem 'rails', '~> 6.1.4', '>= 6.1.4.1'
# Use sqlite3 as the database for Active Record
gem 'sqlite3', '~> 1.4'
# Use Puma as the app server
gem 'puma', '~> 5.0'
# Use SCSS for stylesheets
gem 'sass-rails', '>= 6'
# Transpile app-like JavaScript. Read more: https://github.com/rails/webpacker
gem 'webpacker', '~> 5.0'
# Turbolinks makes navigating your web application faster. Read more: https://github.com/turbolinks/turbolinks
gem 'turbolinks', '~> 5'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '~> 2.7'
# Use Redis adapter to run Action Cable in production
# gem 'redis', '~> 4.0'
# Use Active Model has_secure_password
# gem 'bcrypt', '~> 3.1.7'

# Use Active Storage variant
# gem 'image_processing', '~> 1.2'

# Reduces boot times through caching; required in config/boot.rb
gem 'bootsnap', '>= 1.4.4', require: false

gem 'omniauth-google-oauth2'

group :development, :test do
  # Call 'byebug' anywhere in the code to stop execution and get a debugger console
  gem 'byebug', platforms: [:mri, :mingw, :x64_mingw]
  gem 'dotenv-rails'
end

group :development do
  # Access an interactive console on exception pages or by calling 'console' anywhere in the code.
  gem 'web-console', '>= 4.1.0'
  # Display performance information such as SQL time and flame graphs for each request in your browser.
  # Can be configured to work on production as well see: https://github.com/MiniProfiler/rack-mini-profiler/blob/master/README.md
  gem 'rack-mini-profiler', '~> 2.0'
  gem 'listen', '~> 3.3'
  # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
  gem 'spring'
  gem 'better_errors'
end

group :test do
  # Adds support for Capybara system testing and selenium driver
  gem 'capybara', '>= 3.26'
  gem 'selenium-webdriver'
  # Easy installation and use of web drivers to run system tests with browsers
  gem 'webdrivers'
end

# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]

■config/routes.rb

Rails.application.routes.draw do
  root "posts#index"
  get 'auth/:provider/callback', to: 'sessions#create'
  get 'auth/failure', to: redirect('/')
  get 'signout', to: 'sessions#destroy', as: 'signout'

  resources :sessions, only: %i(new create destroy)
  resources :posts, only: %i(index)
end

■app/models/user.rb

class User < ApplicationRecord
  def self.from_omniauth(auth)
    where(provider: auth.provider, uid: auth.uid).first_or_initialize.tap do |user|
      user.provider = auth.provider
      user.uid = auth.uid
      user.name = auth.info.name
      user.email = auth.info.email
      user.token = auth.credentials.token
      user.expires_at = Time.at(auth.credentials.expires_at)
      return user
    end
  end
end

■app/controllers/sessions_controller.rb

class SessionsController < ApplicationController
  def new
  end

  def create
    user_data = request.env["omniauth.auth"]
    user = User.from_omniauth(request.env["omniauth.auth"])
    if user.save
      session[:user_id] = user.id
      redirect_to root_path
    else
      redirect_to new_session_path
    end
  end

  def destroy
    session[:user_id] = nil
    redirect_to new_session_path
  end
end

■app/controllers/application_controller.rb

class ApplicationController < ActionController::Base
  helper_method :current_user

  def current_user
    User.find(session[:user_id]) if session[:user_id]
  end
end

■app/views/posts/index.html.erb

<h1>Post#index</h1>
<p>Find me in app/views/posts/index.html.erb</p>
<%= link_to "Sign in with Google", "/auth/google_oauth2", id: "sign_in" %>

■config/initializers/omniauth.rb

Rails.application.config.middleware.use OmniAuth::Builder do
  provider :google_oauth2, ENV["GOOGLE_CLIENT_ID"], ENV["GOOGLE_CLIENT_SECRET"]
end

■.env

GOOGLE_CLIENT_ID="======================"
GOOGLE_CLIENT_SECRET="=================="