Following the release of iOS 12 on September 17, Apple has stopped signing iOS 11.41, the previous version of iOS that was available to consumers.
iPhone, iPad, and iPod touch owners who have upgraded to iOS 12 will no longer be able to downgrade to the iOS 11 operating system.
Apple routinely stops signing older versions of software updates after new releases come out in order to encourage customers to keep their operating systems up to date.
iOS 12 and today's iOS 12.0.1 release are the only versions of iOS that can be installed on iPhones and iPads by the general public. Developers and public beta testers can download iOS 12.1, an iOS 12 update that's being beta tested.