Following the release of iOS 13.6 on July 15, Apple has stopped signing iOS 13.5.1, which means downgrading to that version of iOS is no longer possible.
iOS 13.5.1 was a security update that was released to address the vulnerability that allowed for the unc0ver jailbreaking tool that worked with many recent versions of iOS, including iOS 13.5.
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, which introduced Car Keys, Apple News audio stories, and more, 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.