下記エラーでRailsのインストールが出来なかった！！
To see why this extension failed to compile, please check the mkmf.log which can be found here:
/Users/macintosh/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/extensions/x86_64-darwin-17/2.7.0/nokogiri-1.10.10/mkmf.log
extconf failed, exit code 1
下記の設定で解決しました。
gem install nokogiri -- --with-xml2-include=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/usr/include/libxml2 --use-system-libraries