Jekyll_admin_monkey_patch': uninitialized constant Rack::Handler (NameError)

[Mr0Wido]

I stuck with this error for two days and I can't solve it. I need help.
If ı don't use jekyll_admin, bundle exec jekyll serve works correctly. But when I added the jekyll_admin gem, got broke.

/var/lib/gems/3.1.0/gems/jekyll-admin-0.11.1/lib/jekyll/commands/serve.rb:23:in `jekyll_admin_monkey_patch': uninitialized constant Rack::Handler (NameError)

          @server.mount "/admin", Rack::Handler::WEBrick, JekyllAdmin::StaticServer
                                      ^^^^^^^^^
        from /var/lib/gems/3.1.0/gems/jekyll-admin-0.11.1/lib/jekyll/commands/serve.rb:15:in `start_up_webrick'
        from /var/lib/gems/3.1.0/gems/jekyll-4.4.1/lib/jekyll/commands/serve.rb:102:in `process'
        from /var/lib/gems/3.1.0/gems/jekyll-4.4.1/lib/jekyll/command.rb:91:in `block in process_with_graceful_fail'
        from /var/lib/gems/3.1.0/gems/jekyll-4.4.1/lib/jekyll/command.rb:91:in `each'
        from /var/lib/gems/3.1.0/gems/jekyll-4.4.1/lib/jekyll/command.rb:91:in `process_with_graceful_fail'
        from /var/lib/gems/3.1.0/gems/jekyll-4.4.1/lib/jekyll/commands/serve.rb:85:in `block (2 levels) in init_with_program'
        from /var/lib/gems/3.1.0/gems/mercenary-0.4.0/lib/mercenary/command.rb:221:in `block in execute'
        from /var/lib/gems/3.1.0/gems/mercenary-0.4.0/lib/mercenary/command.rb:221:in `each'
        from /var/lib/gems/3.1.0/gems/mercenary-0.4.0/lib/mercenary/command.rb:221:in `execute'
        from /var/lib/gems/3.1.0/gems/mercenary-0.4.0/lib/mercenary/program.rb:44:in `go'
        from /var/lib/gems/3.1.0/gems/mercenary-0.4.0/lib/mercenary.rb:21:in `program'
        from /var/lib/gems/3.1.0/gems/jekyll-4.4.1/exe/jekyll:15:in `<top (required)>'
        from /home/wido/gems/bin/jekyll:25:in `load'
        from /home/wido/gems/bin/jekyll:25:in `<top (required)>'
        from /usr/share/rubygems-integration/all/gems/bundler-2.6.3/lib/bundler/cli/exec.rb:59:in `load'
        from /usr/share/rubygems-integration/all/gems/bundler-2.6.3/lib/bundler/cli/exec.rb:59:in `kernel_load'
        from /usr/share/rubygems-integration/all/gems/bundler-2.6.3/lib/bundler/cli/exec.rb:23:in `run'
        from /usr/share/rubygems-integration/all/gems/bundler-2.6.3/lib/bundler/cli.rb:452:in `exec'
        from /usr/share/rubygems-integration/all/gems/bundler-2.6.3/lib/bundler/vendor/thor/lib/thor/command.rb:28:in `run'
        from /usr/share/rubygems-integration/all/gems/bundler-2.6.3/lib/bundler/vendor/thor/lib/thor/invocation.rb:127:in `invoke_command'
        from /usr/share/rubygems-integration/all/gems/bundler-2.6.3/lib/bundler/vendor/thor/lib/thor.rb:538:in `dispatch'
        from /usr/share/rubygems-integration/all/gems/bundler-2.6.3/lib/bundler/cli.rb:35:in `dispatch'
        from /usr/share/rubygems-integration/all/gems/bundler-2.6.3/lib/bundler/vendor/thor/lib/thor/base.rb:584:in `start'
        from /usr/share/rubygems-integration/all/gems/bundler-2.6.3/lib/bundler/cli.rb:29:in `start'
        from /usr/share/rubygems-integration/all/gems/bundler-2.6.3/exe/bundle:28:in `block in <top (required)>'
        from /usr/share/rubygems-integration/all/gems/bundler-2.6.3/lib/bundler/friendly_errors.rb:117:in `with_friendly_errors'
        from /usr/share/rubygems-integration/all/gems/bundler-2.6.3/exe/bundle:20:in `<top (required)>'
        from /usr/local/bin/bundle:25:in `load'
        from /usr/local/bin/bundle:25:in `<main>'

[thatnerdnoah]

I am getting this exact same error. Not sure what's causing it.

[HONEST-ZH]

so it is the same with me

[HONEST-ZH]

I guess that I find the solution to this problem. Just add this in gemfile gem "sinatra", ">= 3", "< 4".
This problem is because the dependencies of jekyll-admin. Among them, the version of sinatra should not be updated to 4.0 because it dropped support for rack 2.
See more at here