Following the release of iOS 11.4
on May 29, Apple has stopped signing iOS 11.3.1, the previous version of iOS that was available to consumers.
iPhone, iPad, and iPod touch owners who have upgraded to iOS 11.4 will no longer be able to downgrade to earlier versions of iOS.
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 11.4 is now the only version of iOS 11 that can be installed on iOS devices by the general public, but developers can download iOS 12, the next-generation version of iOS that's in beta testing ahead of a fall release.