Following the release of iOS 12.3 on May 13, Apple has stopped signing iOS 12.2, the previous version of iOS that was available to consumers. 

iPhone, iPad, and iPod touch owners who have upgraded to iOS 12.3 will no longer be able to downgrade to iOS 12.2. 

iOS 12.3, available on all devices able to run iOS 12, was a major update that brought the revamped Apple TV app with new Channels feature. 

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 12.3 is now the only version of iOS that can be installed on iPhones and iPads, but developers and public beta testers can download iOS 12.4, another major update that's currently being beta tested.

Source: macrumors