I have a same issue. Log.txt Encountered an error communicating with IBAgent-iOS. (Failure reason: IBAgent-iOS (-1) failed to launch): Failed to launch IBAgent-iOS via CoreSimulator spawn (Failure reason: Failed to spawn IBAgent-iOS on iPhone Xʀ (F2312C83-F9CB-4D88-887A-D8CDBA090B46, iOS 14.3, Booted)): Invalid device state

I downgraded to Xcode 12.2. It works for me. https://developer.apple.com/download/more/?=xcode

clean build, clean derived, restart Xcode/Mac, reinstall Xcode, downgrade to 12.2 doesn't help. I'm running Xcode in Downloads directory, not Application Folder. It may be related to problem. try it.

Check Target Membership of .intentdefinition file in your Widget Extension.