I did pod install for firebase/crashlytics. Now getting error like this on xcode 9.4. I have xcode 11.3 also, on which it works fine.
The latest Firebase release (6.24.0) requires at least Xcode 10.3.
See https://firebase.google.com/docs/ios/setup
You would need to go back to an earlier Firebase version for Xcode 9.
Search for Xcode to get more specifics in the release notes.