The answer for this can be found here here although it is unrelated to this question. Below is a snippet of the answer which works best.
For Xcode 11 on macOS 10.14, this can happen even after installing Xcode and installing command-line tools and accepting the license with
sudo xcode-select --install
sudo xcodebuild -license accept
The issue is that Xcode 11 ships the macOS 10.15 SDK which includes headers for ruby2.6, but not for macOS 10.14's ruby2.3. You can verify that this is your problem by running
ruby -rrbconfig -e 'puts RbConfig::CONFIG["rubyhdrdir"]'
which on macOS 10.14 with Xcode 11 prints the non-existent path
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/include/ruby-2.3.0
However, Xcode 11 installs a macOS 10.14 SDK within /Library/Developer/CommandLineTools/SDKs/MacOS10.14.sdk. It isn't necessary to pollute the system directories by installing the old header files as suggested in other answers. Instead, by selecting that SDK, the appropriate ruby2.3 headers will be found:
sudo xcode-select --switch /Library/Developer/CommandLineTools
ruby -rrbconfig -e 'puts RbConfig::CONFIG["rubyhdrdir"]'
This should now correctly print
/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/include/ruby-2.3.0
Likewise, gem install should work while that SDK is selected.
To switch back to using the current Xcode 11 SDK, use
sudo xcode-select --switch /Applications/Xcode.app
I fixed it by running
brew install cocoapods
And
You may want to remove it:
rm '/usr/local/bin/pod'
To force the link and overwrite all conflicting files:
brew link --overwrite cocoapods
Install homebrew if you haven't already. https://brew.sh/
Best Answer
Open Terminal
Reopen Terminal
From MahmoudKhaled's comment on link