herokuアップ時エラー

ー中略ー
remote:        Bundle complete! 25 Gemfile dependencies, 72 gems now installed.        
remote:        Gems in the groups development and test were not installed.        
remote:        Bundled gems are installed into ./vendor/bundle.        
remote:        Post-install message from sass:        
remote:                
remote:        Ruby Sass is deprecated and will be unmaintained as of 26 March 2019.        
remote:                
remote:        * If you use Sass as a command-line tool, we recommend using Dart Sass, the new        
remote:          primary implementation: https://sass-lang.com/install        
remote:                
remote:        * If you use Sass as a plug-in for a Ruby web framework, we recommend using the        
remote:          sassc gem: https://github.com/sass/sassc-ruby#readme        
remote:                
remote:        * For more details, please refer to the Sass blog:        
remote:          http://sass.logdown.com/posts/7081811        
remote:                
remote:        Bundle completed (47.91s)        
remote:        Cleaning up the bundler cache.        
remote:        Warning: the running version of Bundler (1.15.2) is older than the version that created the lockfile (1.17.0). We suggest you upgrade to the latest version of Bundler by running `gem install bundler`.        
remote:        The latest bundler is 1.17.1, but you are currently running 1.15.2.        
remote:        To update, run `gem install bundler`        
remote: -----> Detecting rake tasks        
remote: -----> Preparing app for Rails asset pipeline        
remote:        Running: rake assets:precompile        
remote:        Yarn executable was not detected in the system.        
remote:        Download Yarn at https://yarnpkg.com/en/docs/install 
ー中略ー
remote:        Tasks: TOP => assets:precompile        
remote:        (See full trace by running task with --trace)        
remote:         
remote:  !        
remote:  !     Precompiling assets failed.        
remote:  !        
remote:  !     Push rejected, failed to compile Ruby app.        
remote: 
remote:  !     Push failed        
remote: Verifying deploy...        
remote: 

文字数の関係上、エラー文を中略しました。
asset:precompileでエラーが起こっているのでしょうか。
どの部分でエラーが起こっているのか、ご教授いただきたいです。

下記記事に書いてあることはしたのですが、それでも治らないです。
https://qiita.com/Oakbow/items/1565922ddcdea0ce9ab5

よろしくお願いします。。。