Apple today has officially stopped signing iOS 12.0.1, which was originally released back at the beginning of October. This means that downgrading from the most recent version of iOS, which is iOS 12.1, is no longer possible.
Apple regularly stops signing older versions of iOS as part of its efforts to keep users on the most up-to-date and therefore secure versions of the operating system. Apple most recently stopped signing iOS 11.4.1, preventing downgrades from iOS 12 to iOS 11, and stopped signing iOS 12.0.
Källa: Apple stops signing iOS 12.0.1 following release of iOS 12.1 & 12.1.1 beta testing
