I put Bootstrap in Rails6. (Sprockets :: Rails :: Helper :: AssetNotPrecompiled handling)

Added to Gemfile

Gemfile

gem 'uglifier'
gem 'jquery-rails'
gem 'bootstrap'

bundle install

Make sure the JS file is compressed in the production environment.

/config/environments/production.rb

config.assets.js_compressor = :uglifier

Create a directory: javascripts under assets, create a new application.js and add it.

app/assets/javascripts/application.js

//= require jquery3
//= require popper
//= require bootstrap

Rename application.css under stylesheets to application.scss and add.

app/assets/stylesheets/application.scss

@import "bootstrap";

Load into layout

html:app/views/layouts/application.html.erb

<%= stylesheet_link_tag 'application', media: 'all', 'data-turbolinks-track': 'reload' %>
<%= javascript_include_tag 'application', 'data-turbolinks-track': 'reload' %>
<%= javascript_pack_tag 'application', 'data-turbolinks-track': 'reload' %>

In my environment, I got an error saying Sprockets :: Rails :: Helper :: AssetNotPrecompiled, so I added it.

config/initializers/assets.rb

Rails.application.config.assets.precompile += %w( application.js )

You have now confirmed that Bootstrap has been applied.

• References https://qiita.com/kazutosato/items/d47b7705ee545de4cb1a