https://wccftech.com/you-can-no-longer-downgrade-to-ios-13-6-or-ipados-13-6/

Apple has stopped signing iOS 13.6 and iPadOS 13.6, meaning you can no longer downgrade from the recently released iOS 13.6.1, iPadOS 13.6.1.

Released just a week ago, iOS 13.6.1 and iPadOS 13.6.1 are nothing more than bug fix updates. If you are looking for new features, you won’t find them here. And since this is a new update, which means it was only a matter of time Apple was going to stop signing iOS 13.6 and iPadOS 13.6, cutting off the process of downgrading to said firmware altogether. Today is that unlucky day and Apple is no longer signing iOS 13.6 and iPadOS 13.6.