I am getting the following error when building in Xcode 10.0 with swift 4.2:
If your file(s) - that couldn't find - icon color is pale. And you right click on it and "Show in Finder" does not open Finder. It means your file name is not same as what you see in the Navigator.
to Solve it, go to where your file exist, and change its name to be same as what you see in the Navigator.