As noted by MacRumors, Apple has stopped signing iOS 16.6, the previously available version of iOS. 

This means that now that iOS 16.6 is no longer being signed, you can’t downgrade to that version of iOS after upgrading to iOS 16.6.1. Apple commonly stops signing older versions of iOS after new releases come out to encourage customers to keep their operating systems up to date.

Article provided with permission from AppleWorld.Today