【Rails】unicornの起動ができない。

前提・実現したいこと

unicornの起動をしたい。

####環境
MacOS Catalina 10.15.7
ruby on rails 6.0.3.4
ruby 2.6.5

発生している問題・エラーメッセージ

Railsを起動しようとしても「masterは起動失敗します。」と出る。

$ bundle exec unicorn_rails -c config/unicorn.rb -E production -D
master failed to start, check stderr log for details

$ RAILS_SERVE_STATIC_FILES=1 unicorn_rails -c config/unicorn.rb -E production -D
master failed to start, check stderr log for details

###ファイル等

ruby:unicorn.rb
1#サーバ上でのアプリケーションコードが設置されているディレクトリを変数に入れておく
2app_path = File.expand_path('../../', __FILE__)
3
4#アプリケーションサーバの性能を決定する
5worker_processes 1
6
7#アプリケーションの設置されているディレクトリを指定
8working_directory app_path
9
10#Unicornの起動に必要なファイルの設置場所を指定
11pid "#{app_path}/tmp/pids/unicorn.pid"
12
13#ポート番号を指定
14listen 3000
15
16#エラーのログを記録するファイルを指定
17stderr_path "#{app_path}/log/unicorn.stderr.log"
18
19#通常のログを記録するファイルを指定
20stdout_path "#{app_path}/log/unicorn.stdout.log"
21
22#Railsアプリケーションの応答を待つ上限時間を設定
23timeout 60
24
25#以下は応用的な設定なので説明は割愛
26
27preload_app true
28GC.respond_to?(:copy_on_write_friendly=) && GC.copy_on_write_friendly = true
29
30check_client_connection false
31
32run_once = true
33
34before_fork do |server, worker|
35  defined?(ActiveRecord::Base) &&
36    ActiveRecord::Base.connection.disconnect!
37
38  if run_once
39    run_once = false # prevent from firing again
40  end
41
42  old_pid = "#{server.config[:pid]}.oldbin"
43  if File.exist?(old_pid) && server.pid != old_pid
44    begin
45      sig = (worker.nr + 1) >= server.worker_processes ? :QUIT : :TTOU
46      Process.kill(sig, File.read(old_pid).to_i)
47    rescue Errno::ENOENT, Errno::ESRCH => e
48      logger.error e
49    end
50  end
51end
52
53after_fork do |_server, _worker|
54  defined?(ActiveRecord::Base) && ActiveRecord::Base.establish_connection
55end

試したこと

エラーメッセージを表示しようとコマンド入力

$ less log/unicorn.stderr.log

Zeitwerk::NameError: expected file /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/devise-4.7.3/app/mailers/devise/mailer.rb to def
ine constant Devise::Mailer, but didn't
  /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/loader/callbacks.rb:17:in `on_file_autoloaded'
  /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/kernel.rb:28:in `block in require'
  /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/kernel.rb:27:in `tap'
  /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/kernel.rb:27:in `require'
  /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/loader.rb:392:in `const_get'
  /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/loader.rb:392:in `block (2 levels) in eager_load'
  /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/loader.rb:742:in `block in ls'

省略

I, [2020-11-13T11:09:42.167780 #3142]  INFO -- : Refreshing Gem list
/home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/loader/callbacks.rb:17:in `on_file_autoloaded': expecte
d file /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/devise-4.7.3/app/mailers/devise/mailer.rb to define constant Devise::Mailer,
 but didn't (Zeitwerk::NameError)
        from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/kernel.rb:28:in `block in require'
        from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/kernel.rb:27:in `tap'
        from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/kernel.rb:27:in `require'
        from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/loader.rb:392:in `const_get'
        from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/loader.rb:392:in `block (2 levels) in eager_load'
        from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/loader.rb:742:in `block in ls'
        from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/loader.rb:734:in `foreach'
        from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/loader.rb:734:in `ls'

省略

I, [2020-11-13T11:18:48.450711 #3285]  INFO -- : Refreshing Gem list
/home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/loader/callbacks.rb:17:in `on_file_autoloaded': expected file /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/devise-4.7.3/app/mailers/devise/mailer.rb to define constant Devise::Mailer, but didn't (Zeitwerk::NameError)
        from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/kernel.rb:28:in `block in require'
        from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/kernel.rb:27:in `tap'
        from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/kernel.rb:27:in `require'
        from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/loader.rb:392:in `const_get'
        from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/loader.rb:392:in `block (2 levels) in eager_load'
        from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/loader.rb:742:in `block in ls'
        from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/loader.rb:734:in `foreach'
        from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/loader.rb:734:in `ls'
        from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/loader.rb:387:in `block in eager_load'
        from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/zeitwerk-2.4.0/lib/zeitwerk/loader.rb:376:in `synchronize'

省略

mailer.rb

if defined?(ActionMailer)
  class Devise::Mailer < Devise.parent_mailer.constantize
    include Devise::Mailers::Helpers

    def confirmation_instructions(record, token, opts = {})
      @token = token
      devise_mail(record, :confirmation_instructions, opts)
    end

    def reset_password_instructions(record, token, opts = {})
      @token = token
      devise_mail(record, :reset_password_instructions, opts)
    end

    def unlock_instructions(record, token, opts = {})
      @token = token
      devise_mail(record, :unlock_instructions, opts)
    end

    def email_changed(record, opts = {})
      devise_mail(record, :email_changed, opts)
    end

    def password_change(record, opts = {})
      devise_mail(record, :password_change, opts)
    end
  end
end

Devise::Mailerが定義されないため止まっていると書いてありますが、定義するにはどうすればいいんでしょうか・・・。
お力添えいただける方がいましたらよろしくお願い致します！