After its release back in September, Apple is no longer signing the initial build of iOS 12. This means that users will now only be able to restore to the current release of iOS, iOS 12.0.1.
iOS 12.0.1 , which was released earlier this month, focused on bug fixes and performance improvements. The update included a fix for the iPhone XS charging issue, Wi-Fi issues, and more.
The next revision of iOS, iOS 12.1, is currently in beta for both developers and public beta testers. It includes support for Group FaceTime, dual SIM with eSIM, and a slew of new emojis. We expect iOS 12.1 to be released later this month or sometime early next month.
It’s normal for Apple to stop signing older versions of iOS as it releases newer versions, so this comes as no surprise. This prevents users from running older, more unstable versions of the operating system, and ensures users are on the most up-to-date and secure release.
Source: 9to5mac