error: Unable to resolve build file: XCBCore.BuildFile

For me, XCode > Product > Clean Build Folder and restart Xcode then worked.

For XCode 10 Beta: Problem occurs when I have two workspaces open that share the same project directories. Solution was

Short version

1. Shut down all but one workspaces
2. exit XCode and reopen XCode
3. XCode > Product > Clean Build Folder

Longer version

1. Shut down all but one workspace
2. XCode > Preferences > Locations > Derived Data > goto directory ~/Library/Developer/Xcode/DerivedData
3. Clear out subdirectories from DerivedData
4. exit XCode and reopen XCode
5. XCode > Product > Clean Build Folder

Thanks to @aferriss answer for the clue.

This was the culprit for my experience of the same error:

After setting the correct version of Command Line Tools (i.e., Xcode 10.2.1; see below) on my system, the error went away!

I came across this same error while using openframeworks 0.10 with xcode 10 recently. It seems like it was caused by having multiple openframeworks projects opened at a time. If you close them all one by one, and then reopen the one you'd like to work on, it should resolve the error. Hoping this is just an xcode-beta thing.

Xcode used to tell you that there as a workspace integrity problem, and you could just ignore it. But it seems like it's returned. There's some discussion about this issue here.