Herokuへのデプロイができない。「Could not detect rake tasks」「pre-receive hook declined」と表示される。

前提・実現したいこと

Railsチュートリアルの2章を進めており、
Herokuへのデプロイを行いたいです。

発生している問題・エラーメッセージ

下記のようなエラーメッセージが表示されており、
デプロイが上手くいきません。

remote:  !
remote:  !     Could not detect rake tasks
remote:  !     ensure you can run `$ bundle exec rake -P` against your app
remote:  !     and using the production group of your Gemfile.
remote:  !     rake aborted!
remote:  !     Unknown version "5.2"
remote:  !     /tmp/build_872bfc3046e51aa71462896c66534052/vendor/bundle/ruby/2.6.0/gems/railties-5.1.6/lib/rails/application/configuration.rb:81:in `load_defaults'
remote:  !     /tmp/build_872bfc3046e51aa71462896c66534052/config/application.rb:12:in `<class:Application>'
remote:  !     /tmp/build_872bfc3046e51aa71462896c66534052/config/application.rb:10:in `<module:ToyApp>'
remote:  !     /tmp/build_872bfc3046e51aa71462896c66534052/config/application.rb:9:in `<top (required)>'
remote:  !     /tmp/build_872bfc3046e51aa71462896c66534052/Rakefile:4:in `require_relative'
remote:  !     /tmp/build_872bfc3046e51aa71462896c66534052/Rakefile:4:in `<top (required)>'
remote:  !     /tmp/build_872bfc3046e51aa71462896c66534052/vendor/bundle/ruby/2.6.0/gems/rake-13.0.0/lib/rake/rake_module.rb:29:in `load'
remote:  !     /tmp/build_872bfc3046e51aa71462896c66534052/vendor/bundle/ruby/2.6.0/gems/rake-13.0.0/lib/rake/rake_module.rb:29:in `load_rakefile'
remote:  !     /tmp/build_872bfc3046e51aa71462896c66534052/vendor/bundle/ruby/2.6.0/gems/rake-13.0.0/lib/rake/application.rb:703:in `raw_load_rakefile'
remote:  !     /tmp/build_872bfc3046e51aa71462896c66534052/vendor/bundle/ruby/2.6.0/gems/rake-13.0.0/lib/rake/application.rb:104:in `block in load_rakefile'
remote:  !     /tmp/build_872bfc3046e51aa71462896c66534052/vendor/bundle/ruby/2.6.0/gems/rake-13.0.0/lib/rake/application.rb:186:in `standard_exception_handling'
remote:  !     /tmp/build_872bfc3046e51aa71462896c66534052/vendor/bundle/ruby/2.6.0/gems/rake-13.0.0/lib/rake/application.rb:103:in `load_rakefile'
remote:  !     /tmp/build_872bfc3046e51aa71462896c66534052/vendor/bundle/ruby/2.6.0/gems/rake-13.0.0/lib/rake/application.rb:82:in `block in run'
remote:  !     /tmp/build_872bfc3046e51aa71462896c66534052/vendor/bundle/ruby/2.6.0/gems/rake-13.0.0/lib/rake/application.rb:186:in `standard_exception_handling'
remote:  !     /tmp/build_872bfc3046e51aa71462896c66534052/vendor/bundle/ruby/2.6.0/gems/rake-13.0.0/lib/rake/application.rb:80:in `run'
remote:  !     /tmp/build_872bfc3046e51aa71462896c66534052/vendor/bundle/ruby/2.6.0/gems/rake-13.0.0/exe/rake:27:in `<top (required)>'
remote:  !     vendor/bundle/bin/rake:29:in `load'
remote:  !     vendor/bundle/bin/rake:29:in `<main>'
remote:  !

試したこと

エラー文を検索し、bundlerのインストールしなおしや、Gemfileの書き換えなどを行いましたが、うまくいきません。

補足情報（FW/ツールのバージョンなど）

エラー文の最下部にはこのような文章が出ています。

 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/protected-ravine-39028.git'

またGemfileは以下の通りです。

source 'https://rubygems.org'

gem 'rails',        '5.1.6'
gem 'puma',         '3.9.1'
gem 'sass-rails',   '5.0.6'
gem 'uglifier',     '3.2.0'
gem 'coffee-rails', '4.2.2'
gem 'jquery-rails', '4.3.1'
gem 'turbolinks',   '5.0.1'
gem 'jbuilder',     '2.7.0'
gem 'bootsnap', require: false

group :development, :test do
  gem 'sqlite3', '1.3.13'
  gem 'byebug',  '9.0.6', platform: :mri
end

group :development do
  gem 'web-console',           '3.5.1'
  gem 'listen',                '3.1.5'
  gem 'spring',                '2.0.2'
  gem 'spring-watcher-listen', '2.0.1'
end

group :production do
  gem 'pg', '0.20.0'
end

# Windows環境ではtzinfo-dataというgemを含める必要があります
gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]

当方初心者のため、何か解決のために必要な情報がありましたら、ご教示ください。
よろしくお願いいたします。