Instalación de cocoapods en MacOS Catalina

En mi mac mini tengo instalado Catalina 10.15.7. También he instalado Xcode 12.4. Ahora como estoy usando una librería de terceros estoy tratando de instalar cocoapods en mi mac. Así que he utilizado este -

sudo gem install cocoapods

que obtuve de oficial sitio web.

Pero estoy recibiendo esto -

Password:
Fetching concurrent-ruby-1.1.8.gem
Fetching fuzzy_match-2.0.4.gem
Fetching i18n-1.8.10.gem
Fetching thread_safe-0.3.6.gem
Fetching tzinfo-1.2.9.gem
Fetching activesupport-5.2.5.gem
Fetching nap-1.1.0.gem
Fetching httpclient-2.8.3.gem
Fetching algoliasearch-1.27.5.gem
Fetching ffi-1.15.0.gem
Fetching ethon-0.12.0.gem
Fetching typhoeus-1.4.0.gem
Fetching netrc-0.11.0.gem
Fetching public_suffix-4.0.6.gem
Fetching addressable-2.7.0.gem
Fetching cocoapods-core-1.10.1.gem
Fetching claide-1.0.3.gem
Fetching cocoapods-deintegrate-1.0.4.gem
Fetching cocoapods-downloader-1.4.0.gem
Fetching cocoapods-plugins-1.0.0.gem
Fetching cocoapods-search-1.0.0.gem
Fetching cocoapods-trunk-1.5.0.gem
Fetching cocoapods-try-1.2.0.gem
Fetching molinillo-0.6.6.gem
Fetching atomos-0.1.3.gem
Fetching colored2-3.1.2.gem
Fetching nanaimo-0.3.0.gem
Fetching xcodeproj-1.19.0.gem
Fetching escape-0.0.4.gem
Fetching fourflusher-2.3.1.gem
Fetching gh_inspector-1.1.3.gem
Fetching ruby-macho-1.4.0.gem
Fetching cocoapods-1.10.1.gem
Successfully installed concurrent-ruby-1.1.8
Successfully installed i18n-1.8.10
Successfully installed thread_safe-0.3.6
Successfully installed tzinfo-1.2.9
Successfully installed activesupport-5.2.5
Successfully installed nap-1.1.0
Successfully installed fuzzy_match-2.0.4
Successfully installed httpclient-2.8.3
A new major version is available for Algolia! Please now use the https://rubygems.org/gems/algolia gem to get the latest features.
Successfully installed algoliasearch-1.27.5
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-1.15.0/ext/ffi_c/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 ./siteconf20210406-1384-1vtmk4p.rb extconf.rb
checking for ffi.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/$(RUBY_BASE_NAME)
    --with-ffi_c-dir
    --without-ffi_c-dir
    --with-ffi_c-include
    --without-ffi_c-include=${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: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:10: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-1.15.0/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Library/Ruby/Gems/2.6.0/gems/ffi-1.15.0 for inspection.
Results logged to /Library/Ruby/Gems/2.6.0/extensions/universal-darwin-19/2.6.0/ffi-1.15.0/gem_make.out

He visto esta pregunta pero mi versión de rubí es -

ruby 2.6.3p62 (2019-04-16 revision 67580) [universal.x86_64-darwin19]

También he tomado una mirada en este pero no pude resolver el problema.

Editar:
He usado esto -

xcode-select --install

pero tengo - xcode-select: error: las herramientas de línea de comandos ya están instaladas, utilice "Software Update" para instalar las actualizaciones

Entonces, ¿hay alguna dependencia de ruby que tenga que instalar primero o me he perdido algo?