Sergei

Спасибо за ответ. Оказалось, не был установлен Git. Но появилась другая напасть - в браузере ошибка 404 Not Found и никаких комментариев. Использую Nginx+Passenger.
Gemfile:
source 'https://rubygems.org'
ruby '2.4.2'
git_source(:github) do |repo_name|
repo_name = "#{repo_name}/#{repo_name}" unless repo_name.include?("/")
"https://github.com/#{repo_name}.git"
end

# June_20
#gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw]

# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '~> 5.1.4'

#June2020 gem 'rails', '~> 5.2.3'
#gem 'rails' 5.2.3

# Use Puma as the app server
gem 'puma', '~> 3.7'
# Use SCSS for stylesheets
gem 'sass-rails', '~> 5.0'
# Use Uglifier as compressor for JavaScript assets
gem 'uglifier', '>= 1.3.0'
# See https://github.com/rails/execjs#readme for more supported runtimes
# gem 'therubyracer', platforms: :ruby

# Use CoffeeScript for .coffee assets and views
gem 'coffee-rails', '~> 4.2'
# Turbolinks makes navigating your web application faster. Read more: https://github.com/turbolinks/turbolinks
#gem 'turbolinks', '~> 5'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '~> 2.5'
# Use Redis adapter to run Action Cable in production
# gem 'redis', '~> 3.0'
# Use ActiveModel has_secure_password
# gem 'bcrypt', '~> 3.1.7'

# Use Capistrano for deployment
# gem 'capistrano-rails', group: :development

gem 'devise'
gem 'pg'
gem 'cancancan', '~> 2.0'
gem 'kaminari'
gem 'ransack', github: 'activerecord-hackery/ransack'
gem 'growlyflash'
gem 'cocoon'
gem 'select2-rails'
gem 'sablon'
gem 'rubyzip', '>= 1.2.1'
gem 'axlsx', git: 'https://github.com/randym/axlsx.git', ref: '776037c0fc799bb09da8c9ea47980bd3bf296874'
gem 'axlsx_rails'
gem 'pandoc-ruby'

group :development, :test do
# Call 'byebug' anywhere in the code to stop execution and get a debugger console
gem 'byebug', platforms: [:mri, :mingw, :x64_mingw]
# Adds support for Capybara system testing and selenium driver
gem 'capybara', '~> 2.13'
gem 'selenium-webdriver'
# Use sqlite3 as the database for Active Record
gem 'sqlite3'
end

group :development do
# Access an IRB console on exception pages or by using <%= console %> anywhere in the code.
gem 'web-console', '>= 3.3.0'
gem 'listen', '>= 3.0.5', '< 3.2'
# Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
gem 'spring'
gem 'spring-watcher-listen', '~> 2.0.0'
gem 'meta_request'
end

# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]