I have an Xcode project which was downloaded from the internet so I don\'t know which Xcode version was used to write it but the language is Swift 2. I opened it in Xcode 7.
Possible solution for those struggling with the above answers.
If you've just restored your machine from a time machine backup (migrating from old to new machine), you may want to re-install Xcode.
It doesn't appear to bring the caches or other important settings across properly from the time machine restore.
I've just taken that; what appears to be a drastic step, but it's worked!
Instructions on how to do so: Completely reinstall Xcode 9
-W