Following the release of iOS 12.4.1 on August 26, Apple has stopped signing iOS 12.4, the previous version of iOS that was available to consumers.
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, but iOS 12.4.1 was released because iOS 12.4 had a major vulnerability.
The vulnerability allowed hackers to create a jailbreak for iOS 12.4 and left devices vulnerable to hacking attempts. Apple fixed the bug in iOS 12.4.1, and the iOS 12.4 jailbreak will not work after upgrading.
iOS 12.4.1 is now the only version of iOS that can be installed on iPhones and iPads, but developers and public beta testers can download iOS 13, an upcoming update that's currently being beta tested and is set to be released next week.