Pages other than index are rendering without sidebar and styling on github pages

[svanschalkwyk]

My landing page renders ok, but any link I click, renders without styling and in a full page.
In _config.yml, I have had to change disable relative permalinks as follows:
# relative_permalinks: true
I also changed the Jekyll version to the latest.
My Gemfile contains:

# This will help ensure the proper Jekyll version is running.
# Happy Jekylling!
#gem "jekyll", "~> 3.9.0"
#gem "jekyll", "~> 4.3.1"

# This is the default theme for new Jekyll sites. You may change this to anything you like.
gem "minima", "~> 2.0"

# If you want to use GitHub Pages, remove the "gem "jekyll"" above and
# uncomment the line below. To upgrade, run `bundle update github-pages`.
gem "github-pages", group: :jekyll_plugins

# If you have any plugins, put them here!
group :jekyll_plugins do
  gem "jekyll-feed", "~> 0.6"
end

# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
# and associated library.
platforms :mingw, :x64_mingw, :mswin, :jruby do
  gem "tzinfo", "~> 1.2"
  gem "tzinfo-data"
end

When I run bundle update github-pages, i get

bundle update github-pages
Could not find gem 'github-pages'.

When I do a bundle install, I get this:

....
Bundler::GemNotFound: Could not find jekyll-sass-converter-1.5.2.gem for installation
...

[LijunLi1211]

I met the same not rendering problem.