Xcode - File not found but build successful

I've seen this too: files that clearly exist get wonky. Oddly, sometimes it fixes itself. It may be an Xcode bug, since the build always succeeds. I've had success with touching the file. sometimes a clean helps. and sometimes quitting xcode helps.... Voodoo really.

You could try to remove the index by deleting everything in ~/Library/Developer/Xcode/DerivedData

That directory is a combined version of the old Build and Intermediates directories from Xcode 1,2 and 3. Basically it's all the temporary and generated files from your project: object files, indexes, etc.

I tried the steps mentioned above, but to fix the error I had to remove the file from "Copy Bundle Resources" in the project "Build Phases".