Cant run assets:precompile on heroku - ruby-on-rails

Question

I am developing a Rails app on Windows 10 under Rails 5.2.1 and Ruby 2.4.4p296
I cannot run the command heroku run rake assets:precompile
I get this error:
C:\Users\Jaiel\Desktop\RubyOnRailsForum>heroku run rake assets:precompile --trace
Running rake assets:precompile --trace on ⬢ warm-mountain-15591... up, run.4644 (Free)
** Invoke assets:precompile (first_time)
** Invoke assets:environment (first_time)
** Execute assets:environment
** Invoke environment (first_time)
** Execute environment
** Invoke yarn:install (first_time)
** Execute yarn:install
Yarn executable was not detected in the system.
Download Yarn at https://yarnpkg.com/en/docs/install
** Execute assets:precompile
Is it even an error? There are assets in the public/assets folder on heroku, however the JavaScript files seem to be not working or are conflicting each other as I also have a problem with them. For example links with method: :delete dont work in my App on Heroku in production environment.
Here is a thread about my problem that is maybe related to it

Answer 1

Try this
$rake assets:precompile (RAILS_ENV=production bundle exec rake assets:precompile is the exact rake task)
Since precompilation is done in production mode only, no need to explicitly specify the environment.
Update:
Try adding the below line to your Gemfile:
group :assets do
gem 'therubyracer'
gem 'sass-rails', " ~> 3.1.0"
gem 'coffee-rails', "~> 3.1.0"
gem 'uglifier'
end
Then run bundle install.
Hope it will work :)