https://wccftech.com/apple-has-completely-killed-the-unc0ver-jailbreak-as-it-stops-signing-ios-13-5/

Apple has stopped signing iOS 13.5 / iPadOS 13.5, which means you can’t downgrade to the firmware and jailbreak if you are already on iOS 13.5.1 or iPadOS 13.5.1.

Having released iOS 13.5.1 and iPadOS 13.5.1 just a few days ago which patches the Unc0ver jailbreak, it was only a matter of time Apple would stop signing the older firmware, killing the process of downgrading from the newer firmware to the older one altogether. In other words, if you made a mistake updating to the new firmware only to downgrade in the hopes for a jailbreak, you can’t do that anymore.