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-2.5.4/ext/nio4r
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/bin/ruby -I /Library/Ruby/Site/2.6.0 -r ./siteconf20201125-6423-amsuu4.rb extconf.rb
checking for unistd.h... *** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
        --with-opt-dir
        --without-opt-dir
        --with-opt-include
        --without-opt-include=${opt-dir}/include
        --with-opt-lib
        --without-opt-lib=${opt-dir}/lib
        --with-make-prog
        --without-make-prog
        --srcdir=.
        --curdir
        --ruby=/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/bin/$(RUBY_BASE_NAME)
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:467:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
        from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:585:in `block in try_compile'
        from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:534:in `with_werror'
        from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:585:in `try_compile'
        from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:1109:in `block in have_header'
        from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:959:in `block in checking_for'
        from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:361:in `block (2 levels) in postpone'
        from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:331:in `open'
        from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:361:in `block in postpone'
        from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:331:in `open'
        from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:357:in `postpone'
        from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:958:in `checking_for'
        from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:1108:in `have_header'
        from extconf.rb:15:in `<main>'

To see why this extension failed to compile, please check the mkmf.log which can be found here:

  /Library/Ruby/Gems/2.6.0/extensions/universal-darwin-19/2.6.0/nio4r-2.5.4/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Library/Ruby/Gems/2.6.0/gems/nio4r-2.5.4 for inspection.
Results logged to /Library/Ruby/Gems/2.6.0/extensions/universal-darwin-19/2.6.0/nio4r-2.5.4/gem_make.out




#  /Library/Ruby/Gems/2.6.0/extensions/universal-darwin-19/2.6.0/nio4r-2.5.4/mkmf.log
"xcrun clang -o conftest -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.0.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/universal-darwin19 -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.0.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/ruby/backward -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.0.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0 -I. -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -D_DARWIN_UNLIMITED_SELECT -D_REENTRANT    -g -Os -pipe -DHAVE_GCC_ATOMIC_BUILTINS conftest.c  -L. -L/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib -L. -L/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.Internal.sdk/usr/local/lib   -arch x86_64   -lruby.2.6   "
In file included from conftest.c:1:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.0.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/ruby.h:33:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.0.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/ruby/ruby.h:24:10: fatal error: 'ruby/config.h' file not found
#include "ruby/config.h"
         ^~~~~~~~~~~~~~~
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.0.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/ruby/ruby.h:24:10: note: did not find header 'config.h' in framework 'ruby' (loaded from '/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/System/Library/Frameworks')
1 error generated.
checked program was:
/* begin */
1: #include "ruby.h"
2:
3: int main(int argc, char **argv)
4: {
5:   return 0;
6: }
/* end */