Xcode – CocoaPods installation failed "Failed to build gem native extension"

I cannot install the cocoapods to my Mac Catalina by the following command:

sudo gem install cocoapods

Here is the error message from the terminal:

Building native extensions. This could take a while...
ERROR:  Error installing cocoapods:
    ERROR: Failed to build gem native extension.

    current directory: /Library/Ruby/Gems/2.6.0/gems/ffi-1.13.1/ext/ffi_c
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/bin/ruby -I /Library/Ruby/Site/2.6.0 -r ./siteconf20201117-5613-1jjoznr.rb extconf.rb
*** 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/$(RUBY_BASE_NAME)
    --with-ffi_c-dir
    --without-ffi_c-dir
    --with-ffi_c-include
    --without-ffi_c-include=${ffi_c-dir}/include
    --with-ffi_c-lib
    --without-ffi_c-lib=${ffi_c-dir}/lib
    --enable-system-libffi
    --disable-system-libffi
    --with-libffi-config
    --without-libffi-config
    --with-pkg-config
    --without-pkg-config
/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:546:in `block in try_link0'
    from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/tmpdir.rb:93:in `mktmpdir'
    from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:543:in `try_link0'
    from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:570:in `try_link'
    from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:672:in `try_ldflags'
    from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:1832:in `pkg_config'
    from extconf.rb:9:in `system_libffi_usable?'
    from extconf.rb:42: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/ffi-1.13.1/mkmf.log

extconf failed, exit code 1

Here is the error shown at the /Library/Ruby/Gems/2.6.0/extensions/universal-darwin-19/2.6.0/ffi-1.13.1/mkmf.log:

| pkg-config --libs libffi
=> "-lffin"
"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 */

I have installed the Xcode 12.2(12B45b) to Mac and selected it as the Command Line Tools.
I am very confused with these messages. Thank you very much !!!