td-agentの起動に失敗する

前提・実現したいこと

以下のコマンドでtd-agentをインストールしました。

$ sudo curl -L https://toolbelt.treasuredata.com/sh/install-ubuntu-xenial-td-agent2.sh | sh

発生している問題・エラーメッセージ

td-agentを起動しようとするとエラーが出て失敗してしまいます。

$ sudo systemctl start td-agent.service
Job for td-agent.service failed because the control process exited with error code. See "systemctl status td-agent.service" and "journalctl -xe" for details.
$ sudo systemctl status td-agent.service
● td-agent.service - td-agent: Fluentd based data collector for Treasure Data
   Loaded: loaded (/lib/systemd/system/td-agent.service; disabled; vendor preset: enabled)
   Active: failed (Result: start-limit-hit) since Mon 2020-08-31 08:02:27 UTC; 2min 7s ago
     Docs: https://docs.treasuredata.com/articles/td-agent
  Process: 25841 ExecStart=/opt/td-agent/embedded/bin/fluentd --log $TD_AGENT_LOG_FILE --daemon /var/run/td-agent/td-agent.pid $TD_AGENT_OPTIONS (code=exited, status=1/FAILURE)

Aug 31 08:02:26 ubuntu16-td-agent systemd[1]: td-agent.service: Control process exited, code=exited status=1
Aug 31 08:02:26 ubuntu16-td-agent systemd[1]: Failed to start td-agent: Fluentd based data collector for Treasure Data.
Aug 31 08:02:26 ubuntu16-td-agent systemd[1]: td-agent.service: Unit entered failed state.
Aug 31 08:02:26 ubuntu16-td-agent systemd[1]: td-agent.service: Failed with result 'exit-code'.
Aug 31 08:02:27 ubuntu16-td-agent systemd[1]: td-agent.service: Service hold-off time over, scheduling restart.
Aug 31 08:02:27 ubuntu16-td-agent systemd[1]: Stopped td-agent: Fluentd based data collector for Treasure Data.
Aug 31 08:02:27 ubuntu16-td-agent systemd[1]: td-agent.service: Start request repeated too quickly.
Aug 31 08:02:27 ubuntu16-td-agent systemd[1]: Failed to start td-agent: Fluentd based data collector for Treasure Data.
Aug 31 08:02:27 ubuntu16-td-agent systemd[1]: td-agent.service: Unit entered failed state.
Aug 31 08:02:27 ubuntu16-td-agent systemd[1]: td-agent.service: Failed with result 'start-limit-hit'.

追記
journalctl -xe のログです。

-- Unit td-agent.service has begun starting up.
Aug 31 09:19:44 h fluentd[26826]: /opt/td-agent/embedded/lib/ruby/site_ruby/2.1.0/rubygems.rb:271:in `find_spec_for_exe': can't find gem fluentd (>= 0.a) (Gem::GemNotFoundException)
Aug 31 09:19:44 h fluentd[26826]:         from /opt/td-agent/embedded/lib/ruby/site_ruby/2.1.0/rubygems.rb:299:in `activate_bin_path'
Aug 31 09:19:44 h fluentd[26826]:         from /opt/td-agent/embedded/bin/fluentd:23:in `<main>'
Aug 31 09:19:44 h systemd[1]: td-agent.service: Control process exited, code=exited status=1
Aug 31 09:19:44 h systemd[1]: Failed to start td-agent: Fluentd based data collector for Treasure Data.
-- Subject: Unit td-agent.service has failed
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit td-agent.service has failed.
-- 
-- The result is failed.
Aug 31 09:19:44 h systemd[1]: td-agent.service: Unit entered failed state.
Aug 31 09:19:44 h systemd[1]: td-agent.service: Failed with result 'exit-code'.
Aug 31 09:19:44 h systemd[1]: td-agent.service: Service hold-off time over, scheduling restart.
Aug 31 09:19:44 h systemd[1]: Stopped td-agent: Fluentd based data collector for Treasure Data.
-- Subject: Unit td-agent.service has finished shutting down
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit td-agent.service has finished shutting down.
Aug 31 09:19:44 h systemd[1]: td-agent.service: Start request repeated too quickly.
Aug 31 09:19:44 h systemd[1]: Failed to start td-agent: Fluentd based data collector for Treasure Data.
-- Subject: Unit td-agent.service has failed
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit td-agent.service has failed.
-- 
-- The result is failed.
Aug 31 09:19:44 h systemd[1]: td-agent.service: Unit entered failed state.
Aug 31 09:19:44 h systemd[1]: td-agent.service: Failed with result 'start-limit-hit'.

syslogの出力です。

Aug 31 09:27:39 h systemd[1]: Starting td-agent: Fluentd based data collector for Treasure Data...
Aug 31 09:27:39 h fluentd[27317]: /opt/td-agent/embedded/lib/ruby/site_ruby/2.1.0/rubygems.rb:271:in `find_spec_for_exe': can't find gem fluentd (>= 0.a) (Gem::GemNotFoundException)
Aug 31 09:27:39 h fluentd[27317]: #011from /opt/td-agent/embedded/lib/ruby/site_ruby/2.1.0/rubygems.rb:299:in `activate_bin_path'
Aug 31 09:27:39 h fluentd[27317]: #011from /opt/td-agent/embedded/bin/fluentd:23:in `<main>'
Aug 31 09:27:39 h systemd[1]: td-agent.service: Control process exited, code=exited status=1
Aug 31 09:27:39 h systemd[1]: Failed to start td-agent: Fluentd based data collector for Treasure Data.
Aug 31 09:27:39 h systemd[1]: td-agent.service: Unit entered failed state.
Aug 31 09:27:39 h systemd[1]: td-agent.service: Failed with result 'exit-code'.
Aug 31 09:27:39 h systemd[1]: td-agent.service: Service hold-off time over, scheduling restart.
Aug 31 09:27:39 h systemd[1]: Stopped td-agent: Fluentd based data collector for Treasure Data.
Aug 31 09:27:39 h systemd[1]: Starting td-agent: Fluentd based data collector for Treasure Data...
Aug 31 09:27:39 h fluentd[27322]: /opt/td-agent/embedded/lib/ruby/site_ruby/2.1.0/rubygems.rb:271:in `find_spec_for_exe': can't find gem fluentd (>= 0.a) (Gem::GemNotFoundException)
Aug 31 09:27:39 h fluentd[27322]: #011from /opt/td-agent/embedded/lib/ruby/site_ruby/2.1.0/rubygems.rb:299:in `activate_bin_path'
Aug 31 09:27:39 h fluentd[27322]: #011from /opt/td-agent/embedded/bin/fluentd:23:in `<main>'
Aug 31 09:27:39 h systemd[1]: td-agent.service: Control process exited, code=exited status=1
Aug 31 09:27:39 h systemd[1]: Failed to start td-agent: Fluentd based data collector for Treasure Data.
Aug 31 09:27:39 h systemd[1]: td-agent.service: Unit entered failed state.
Aug 31 09:27:39 h systemd[1]: td-agent.service: Failed with result 'exit-code'.
Aug 31 09:27:40 h systemd[1]: td-agent.service: Service hold-off time over, scheduling restart.
Aug 31 09:27:40 h systemd[1]: Stopped td-agent: Fluentd based data collector for Treasure Data.
Aug 31 09:27:40 h systemd[1]: Starting td-agent: Fluentd based data collector for Treasure Data...
Aug 31 09:27:40 h fluentd[27327]: /opt/td-agent/embedded/lib/ruby/site_ruby/2.1.0/rubygems.rb:271:in `find_spec_for_exe': can't find gem fluentd (>= 0.a) (Gem::GemNotFoundException)
Aug 31 09:27:40 h fluentd[27327]: #011from /opt/td-agent/embedded/lib/ruby/site_ruby/2.1.0/rubygems.rb:299:in `activate_bin_path'
Aug 31 09:27:40 h fluentd[27327]: #011from /opt/td-agent/embedded/bin/fluentd:23:in `<main>'
Aug 31 09:27:40 h systemd[1]: td-agent.service: Control process exited, code=exited status=1
Aug 31 09:27:40 h systemd[1]: Failed to start td-agent: Fluentd based data collector for Treasure Data.
Aug 31 09:27:40 h systemd[1]: td-agent.service: Unit entered failed state.
Aug 31 09:27:40 h systemd[1]: td-agent.service: Failed with result 'exit-code'.
Aug 31 09:27:40 h systemd[1]: td-agent.service: Service hold-off time over, scheduling restart.
Aug 31 09:27:40 h systemd[1]: Stopped td-agent: Fluentd based data collector for Treasure Data.
Aug 31 09:27:40 h systemd[1]: Starting td-agent: Fluentd based data collector for Treasure Data...
Aug 31 09:27:40 h fluentd[27332]: /opt/td-agent/embedded/lib/ruby/site_ruby/2.1.0/rubygems.rb:271:in `find_spec_for_exe': can't find gem fluentd (>= 0.a) (Gem::GemNotFoundException)
Aug 31 09:27:40 h fluentd[27332]: #011from /opt/td-agent/embedded/lib/ruby/site_ruby/2.1.0/rubygems.rb:299:in `activate_bin_path'
Aug 31 09:27:40 h fluentd[27332]: #011from /opt/td-agent/embedded/bin/fluentd:23:in `<main>'
Aug 31 09:27:40 h systemd[1]: td-agent.service: Control process exited, code=exited status=1
Aug 31 09:27:40 h systemd[1]: Failed to start td-agent: Fluentd based data collector for Treasure Data.
Aug 31 09:27:40 h systemd[1]: td-agent.service: Unit entered failed state.
Aug 31 09:27:40 h systemd[1]: td-agent.service: Failed with result 'exit-code'.
Aug 31 09:27:40 h systemd[1]: td-agent.service: Service hold-off time over, scheduling restart.
Aug 31 09:27:40 h systemd[1]: Stopped td-agent: Fluentd based data collector for Treasure Data.
Aug 31 09:27:40 h systemd[1]: Starting td-agent: Fluentd based data collector for Treasure Data...
Aug 31 09:27:40 h fluentd[27337]: /opt/td-agent/embedded/lib/ruby/site_ruby/2.1.0/rubygems.rb:271:in `find_spec_for_exe': can't find gem fluentd (>= 0.a) (Gem::GemNotFoundException)
Aug 31 09:27:40 h fluentd[27337]: #011from /opt/td-agent/embedded/lib/ruby/site_ruby/2.1.0/rubygems.rb:299:in `activate_bin_path'
Aug 31 09:27:40 h fluentd[27337]: #011from /opt/td-agent/embedded/bin/fluentd:23:in `<main>'
Aug 31 09:27:40 h systemd[1]: td-agent.service: Control process exited, code=exited status=1
Aug 31 09:27:40 h systemd[1]: Failed to start td-agent: Fluentd based data collector for Treasure Data.
Aug 31 09:27:40 h systemd[1]: td-agent.service: Unit entered failed state.
Aug 31 09:27:40 h systemd[1]: td-agent.service: Failed with result 'exit-code'.
Aug 31 09:27:40 h systemd[1]: td-agent.service: Service hold-off time over, scheduling restart.
Aug 31 09:27:40 h systemd[1]: Stopped td-agent: Fluentd based data collector for Treasure Data.
Aug 31 09:27:40 h systemd[1]: td-agent.service: Start request repeated too quickly.
Aug 31 09:27:40 h systemd[1]: Failed to start td-agent: Fluentd based data collector for Treasure Data.
Aug 31 09:27:40 h systemd[1]: td-agent.service: Unit entered failed state.
Aug 31 09:27:40 h systemd[1]: td-agent.service: Failed with result 'start-limit-hit'.