gem can only install sassc <= 2.0.1, macOS

[KKSun]

some times the log is like this

Installing sassc 2.3.0 (was 2.0.1) with native extensions
Errno::EACCES: Permission denied @ rb_sysopen - /Users/sunhlf/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/sassc-2.3.0/.gitignore
An error occurred while installing sassc (2.3.0), and Bundler cannot continue.
Make sure that `gem install sassc -v '2.3.0' --source 'https://rubygems.org/'` succeeds before bundling.

In Gemfile:
  sass-rails was resolved to 6.0.0, which depends on
    sassc-rails was resolved to 2.1.2, which depends on
      sassc

others the logs show

Installing sassc 2.2.1 (was 2.0.1) with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /Users/sunhlf/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/sassc-2.2.1/ext
/Users/sunhlf/.rbenv/versions/2.7.1/bin/ruby -I /Users/sunhlf/.rbenv/versions/2.7.1/lib/ruby/site_ruby/2.7.0 -r
./siteconf20200510-50711-4zsb77.rb extconf.rb
creating Makefile

current directory: /Users/sunhlf/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/sassc-2.2.1/ext
make "DESTDIR=" clean

current directory: /Users/sunhlf/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/sassc-2.2.1/ext
make "DESTDIR="
compiling ./libsass/src/units.cpp
In file included from ./libsass/src/units.cpp:1:
./libsass/src/sass.hpp:55:10: fatal error: 'string' file not found
#include <string>
         ^~~~~~~~
1 error generated.
make: *** [units.o] Error 1

make failed, exit code 2

Gem files will remain installed in /Users/sunhlf/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/sassc-2.2.1 for inspection.
Results logged to /Users/sunhlf/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/extensions/x86_64-darwin-19/2.7.0/sassc-2.2.1/gem_make.out

An error occurred while installing sassc (2.2.1), and Bundler cannot continue.
Make sure that `gem install sassc -v '2.2.1' --source 'https://rubygems.org/'` succeeds before bundling.

In Gemfile:
  sass-rails was resolved to 6.0.0, which depends on
    sassc-rails was resolved to 2.1.2, which depends on
      sassc

seems to be missing compile flags??

[dsounded]

same here