rails unicorn起動しない

Question

本番環境でunicornが起動しなくて困っております。

[ec2-user@ip-10-0-1-60 music_app]$ bundle exec unicorn_rails -c /var/www/rails/music_app/config/unicorn.conf.rb -D -E production
bundler: failed to load command: unicorn_rails (/home/ec2-user/.rbenv/versions/2.5.3/bin/unicorn_rails)
ArgumentError: directory for pid=/var/www/rails/music_app/tmp/pids/unicorn.pid not writable
/home/ec2-user/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/unicorn-5.4.1/lib/unicorn/configurator.rb:100:in `block in reload'
  /home/ec2-user/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/unicorn-5.4.1/lib/unicorn/configurator.rb:96:in `each'
  /home/ec2-user/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/unicorn-5.4.1/lib/unicorn/configurator.rb:96:in `reload'
  /home/ec2-user/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/unicorn-5.4.1/lib/unicorn/configurator.rb:77:in `initialize'
  /home/ec2-user/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/unicorn-5.4.1/lib/unicorn/http_server.rb:77:in `new'
  /home/ec2-user/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/unicorn-5.4.1/lib/unicorn/http_server.rb:77:in `initialize'
  /home/ec2-user/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/unicorn-5.4.1/bin/unicorn_rails:209:in `new'
  /home/ec2-user/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/unicorn-5.4.1/bin/unicorn_rails:209:in `<top (required)>'
  /home/ec2-user/.rbenv/versions/2.5.3/bin/unicorn_rails:23:in `load'
  /home/ec2-user/.rbenv/versions/2.5.3/bin/unicorn_rails:23:in `<top (required)>'
master failed to start, check stderr log for details

権限周りなのかと思ったのですがそういうわけでもなかったです

# set lets
  $worker  = 2
  $timeout = 30
  $app_dir = "/var/www/rails/music_app"
  $listen  = File.expand_path 'tmp/sockets/.unicorn.sock', $app_dir
  $pid     = File.expand_path 'tmp/pids/unicorn.pid', $app_dir
  $std_log = File.expand_path 'log/unicorn.log', $app_dir
  # set config
  worker_processes  $worker
  working_directory $app_dir
  stderr_path $std_log
  stdout_path $std_log
  timeout $timeout
  listen  $listen
  pid $pid
  # loading booster
  preload_app true
  # before starting processes
  before_fork do |server, worker|
    defined?(ActiveRecord::Base) and ActiveRecord::Base.connection.disconnect!
    old_pid = "#{server.config[:pid]}.oldbin"
    if old_pid != server.pid
      begin
        Process.kill "QUIT", File.read(old_pid).to_i
      rescue Errno::ENOENT, Errno::ESRCH
      end
    end
  end
  # after finishing processes
  after_fork do |server, worker|
    defined?(ActiveRecord::Base) and ActiveRecord::Base.establish_connection
  end

もしわかる方がいればご教授いただけないでしょうか?
宜しくお願いします。