At first I was getting an error that involved my signing identity. After signing out and then signing back in, I'm now getting the following errors. I think it has something to do with the the certificates or provisioning profile.
error: A cryptographic verification failure has occurred. *** error: Couldn't codesign /Users/x/Library/Developer/Xcode/DerivedData/SeedDemo-elhrfqwwnjtcgucmplbbubsbcjcd/Build/Products/Debug-iphoneos/SeedDemo.app/Frameworks/libswiftCore.dylib: codesign failed with exit code 1
This is a macOS Sierra problem that is affecting many other developers. Your options now are to revert back to El Cap, or to wait and hope for a resolution with the next Sierra / Xcode 8 beta. I wish there was a better answer, but at this time (6/26/2016), those are the only know solutions.
Some developers are reporting this fixed with macOS Sierra Beta 2 and Xcode 8 Beta 2, but I have not yet comfirmed on my spare Mac if this fixes the problem.
Also, some developers have reported that resetting the Keychain fixes is. In the Keychain Access app , go to "Preferences..." and click "Reset My Default Keychain".