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

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.

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

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