　起こったこと

$ rails test
Running via Spring preloader in process XXXXXX
/home/ec2-user/environment/Sample_Apps/sample_app/db/schema.rb doesn't exist yet. Run `rails db:migrate` to create it, then try again. If you do not intend to use a database, you should instead alter /home/ec2-user/environment/Sample_Apps/sample_app/config/application.rb to limit the frameworks that will be loaded.
Run options: --seed 4673

# Running:

..

Finished in 0.434520s, 4.6028 runs/s, 4.6028 assertions/s.

2 runs, 2 assertions, 0 failures, 0 errors, 0 skips
/usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/tmux/client.rb:12:in `version': undefined method `[]' for nil:NilClass (NoMethodError)
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/tmux.rb:69:in `_check_available'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/base.rb:59:in `initialize'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/detected.rb:100:in `new'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/detected.rb:100:in `_add'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/detected.rb:62:in `block (2 levels) in detect'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/detected.rb:60:in `each'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/detected.rb:60:in `detect'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/detected.rb:60:in `block in detect'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/detected.rb:59:in `each'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier/detected.rb:59:in `detect'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier.rb:180:in `_detect_or_add_notifiers'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier.rb:198:in `_activate'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier.rb:87:in `initialize'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier.rb:42:in `new'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/notiffany-0.1.1/lib/notiffany/notifier.rb:42:in `connect'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/guard-2.13.0/lib/guard/notifier.rb:11:in `connect'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/guard-2.13.0/lib/guard/notifier.rb:31:in `notify'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/guard-compat-1.2.1/lib/guard/compat/plugin.rb:113:in `notify'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/guard-minitest-2.4.4/lib/guard/minitest/notifier.rb:31:in `notify'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/guard-minitest-2.4.4/lib/guard/minitest/reporter.rb:10:in `report'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/minitest-5.10.3/lib/minitest.rb:687:in `each'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/minitest-5.10.3/lib/minitest.rb:687:in `report'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/minitest-5.10.3/lib/minitest.rb:141:in `run'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/minitest-5.10.3/lib/minitest.rb:63:in `block in autorun'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/spring-2.0.2/lib/spring/application.rb:171:in `fork'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/spring-2.0.2/lib/spring/application.rb:171:in `serve'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/spring-2.0.2/lib/spring/application.rb:141:in `block in run'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/spring-2.0.2/lib/spring/application.rb:135:in `loop'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/spring-2.0.2/lib/spring/application.rb:135:in `run'
        from /usr/local/rvm/gems/ruby-2.4.1/gems/spring-2.0.2/lib/spring/application/boot.rb:19:in `<top (required)>'
        from /usr/local/rvm/rubies/ruby-2.4.1/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
        from /usr/local/rvm/rubies/ruby-2.4.1/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
        from -e:1:in `<main>'

対処法

参照URL
https://yskoht.hatenablog.com/entry/2017/12/26/140450

最初のエラー文は、

schema.rb doesn't exist yet. Run `rails db:migrate` to create it, then try again.

とあるので、指示通りマイグレーションを行います。

$ rails db:migrate

これで最初の方のエラーが消えます。

後半のエラーは

$ sudo yum install -y tmux

と、tmuxをインストールすると消えます。

tmuxとは

tmux とは、Unixライクなオペレーティングシステム用の端末多重接続ソフトウェア
https://ja.wikipedia.org/wiki/Tmux

画面一つで複数のターミナルを操作できるソフトのようです。
rails testの際に使われている？ということなんでしょうか。