Following the release of iOS 13.6.1 on August 12, Apple has stopped signing iOS 13.6, which means downgrading to that version of iOS is no longer possible.
iOS 13.6 was a major update that introduced Car Keys support Apple News audio, and other features.
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 13.6.1, a bug fix issue that addressed problems with data storage, thermal management, and exposure notifications, is the only current publicly available version of iOS that can be installed on iPhones and iPads. Apple has also seeded betas of upcoming iOS and iPadOS 14 updates to developers and public beta testers, which can be downloaded instead.