You want to add pod 'GoogleMobileAdsMediationFacebook', '~> 6.21' similar to the following to your Podfile:

target 'MyApp' do
  pod 'GoogleMobileAdsMediationFacebook', '~> 6.21'
end

Then run a pod install inside your terminal, or from CocoaPods.app.

Alternatively to give it a test run, run the command:

pod try GoogleMobileAdsMediationFacebook