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...
Read this story at MacRumors