I got this error even with the new version of Beta6 that was release hours after the bad one got pulled.
I’ve solved this and other similarly illegible errors by deleting the contents of the Derived folder. You can find where that folder is located by going to Preferences > Locations.
The default path is:
You can also hold
Option while the Product menu is open in Xcode, which will change
Clean Build Folder… and accomplish the same task without having to folder-hunt.