iPhone 6s stuck in DFU mode after downgrading
The short answer is to reinstall iOS 11 - the longer discussion and answer are covered here.


iPhone 7 Plus stuck in recovery mode and can&#39;t restore/update even in DFU


It appears either iPhones can't go back at all or they can't go back today with the current version of iOS 10, iOS 11 and/or current version of iTunes. The iPad Air 2 is the only device I've tried to revert back to 10 - that succeeded.

Past that, I could speculate that it's likely this is firmware and/or secure enclave related but I haven't done the engineering work (or seen another release their analysis) yet. For now, I would urge everyone to not install any of APple's current betas on a device they cannot devote to testing and breakage until the presumed fall release. That includes macOS conversion to APFS, iOS, watchOS and tvOS. Apple announced massive changes to these builds and what worked to go back in the past might either not be tested, not functional or intentionally not allowed.