Rename failed in Xcode 9

Try again in few minutes. Maybe background indexing still running. Also, you must build your project in order to be able to use refactor->rename.
Actually, it must be a bug in Xcode 9. Try 9.1 beta, refactor is not ideal, but works better there.
this maybe helpful: https://forums.developer.apple.com/thread/80331

UPDATE:

1. Close Xcode
2. Go to ~/Library/Developer/Xcode/DerivedData and remove folder contents including “ModuleCache”.
3. Open your project and build
4. Try rename functionality now. It should work!

Update 2:

For Xcode 10, it’s enough just to restart it.

Update 3:

Still works for Xcode 12. Sometimes just restart helps, sometimes module cache needs to be removed. Sometimes even build->clean helps.