Railsの環境構築で、sudo gem install railsを実行した時に、次のようなエラーが発生しました。


% sudo gem install rails -v "6.1.3"  
Building native extensions. This could take a while...
ERROR:  Error installing rails:
	ERROR: Failed to build gem native extension.

    current directory: /Library/Ruby/Gems/2.6.0/gems/nio4r-2.5.5/ext/nio4r
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/bin/ruby -I /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0 -r ./siteconf20210227-1015-xk5n2d.rb extconf.rb
mkmf.rb can't find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/include/ruby.h

You might have to install separate package for the ruby development
environment, ruby-dev or ruby-devel for example.

extconf failed, exit code 1

Gem files will remain installed in /Library/Ruby/Gems/2.6.0/gems/nio4r-2.5.5 for inspection.
Results logged to /Library/Ruby/Gems/2.6.0/extensions/universal-darwin-20/2.6.0/nio4r-2.5.5/gem_make.out

この場合、xcode-select --installを実行することで解決しました。

試してみて下さい。