error: Unable to resolve build file: XCBCore.BuildFile

Question

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.

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.

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

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!

Just Restarted my Xcode and the problem went away

The key here is missingTargetProductReference, or you might get namedReferencesCannotBeResolved. Xcode can't find something.

This happened after upgrading to Swift5/Xcode10.

I found that removing linked frameworks and then re-adding them fixed the issue.