UnityでiOSでのビルド時に"Failed to install CocoaPods for the current user."とエラーが起こる。

Unityでビルド時「Failed to install CocoaPods for the current user.」とエラーが起きてしまいます。

UnityでiOSでのビルド後に

gem install cocoapods --user-install
WARNING:  You don't have /Users/jejeje/.gem/ruby/2.6.0/bin in your PATH,
	  gem executables will not run.Building native extensions. This could take a while...
ERROR:  Error installing cocoapods:
	ERROR: Failed to build gem native extension.

Failed to install CocoaPods for the current user.

It will not be possible to install Cocoapods in the generated Xcode project which will result in link errors when building your application.

For more information see:
  https://guides.cocoapods.org/using/getting-started.html


'gem install cocoapods --user-install' failed with code (1):
Building native extensions. This could take a while...


WARNING:  You don't have /Users/jejeje/.gem/ruby/2.6.0/bin in your PATH,
	  gem executables will not run.
ERROR:  Error installing cocoapods:
	ERROR: Failed to build gem native extension.

    current directory: /Users/jejeje/.gem/ruby/2.6.0/gems/ffi-1.14.2/ext/ffi_c
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/bin/ruby -I /Library/Ruby/Site/2.6.0 -r ./siteconf20210207-9261-1k9mm90.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:

  /Users/jejeje/.gem/ruby/2.6.0/extensions/universal-darwin-19/2.6.0/ffi-1.14.2/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Users/jejeje/.gem/ruby/2.6.0/gems/ffi-1.14.2 for inspection.
Results logged to /Users/jejeje/.gem/ruby/2.6.0/extensions/universal-darwin-19/2.6.0/ffi-1.14.2/gem_make.out


UnityEngine.Debug:LogError(Object)
Google.Logger:Log(String, LogLevel) (at /Users/chkuang/Workspace/Git/unity-jar-resolver/source/VersionHandlerImpl/src/Logger.cs:136)
Google.IOSResolver:Log(String, Boolean, LogLevel) (at /Users/chkuang/Workspace/Git/unity-jar-resolver/source/IOSResolver/src/IOSResolver.cs:1159)
Google.<InstallCocoapods>c__AnonStorey2:<>m__6(Int32, CommandItem[], Result, CommandLineDialog) (at /Users/chkuang/Workspace/Git/unity-jar-resolver/source/IOSResolver/src/IOSResolver.cs:1725)
Google.IOSResolver:RunCommandsAsync(CommandItem[], CommandItemCompletionHandler, Boolean, String) (at /Users/chkuang/Workspace/Git/unity-jar-resolver/source/IOSResolver/src/IOSResolver.cs:2388)
Google.IOSResolver:InstallCocoapods(Boolean, String, Boolean) (at /Users/chkuang/Workspace/Git/unity-jar-resolver/source/IOSResolver/src/IOSResolver.cs:1715)
Google.IOSResolver:OnPostProcessEnsurePodsInstallation(BuildTarget, String) (at /Users/chkuang/Workspace/Git/unity-jar-resolver/source/IOSResolver/src/IOSResolver.cs:1813)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr) (at /Users/bokken/buildslave/unity/build/Modules/IMGUI/GUIUtility.cs:197)

といったエラーが発生します。
これは今までは起こらずビルド出来ていたのですが、Google adMobを導入した後から発生するようになりました。

対処方法が知りたいです。よろしくお願いします。