Apple released iOS 13.6 on July 15, and as is traditional, Apple has now stopped signing iOS 13.5.1, making downgrading to that version of iOS no longer possible.
The iOS 13.5.1 update fixed a vulnerability that allowed the unc0ver 5.0 jailbreak that worked on all recent versions of iOS, including version 13.5.
Apple is known to stop signing older versions of operating systems after it releases a public update, as part of an effort to encourage users to keep their devices’ operating systems up to date.