There are times when you have weird Xcode build errors eg. you are absolutely sure that the referenced object is in the project, yet Xcode could not find it.

Or compile errors such as Include of non-modular header inside framework module, when you know the framework works very fine.

You can try clean (Cmd + Shift + K) and build.

But the real way to clean Xcode is to delete folders in ~/Library/Developer/Xcode/DerivedData. Delete folders with your app name, and also ModuleCache. That could well save your day.