https://wccftech.com/apple-just-stopped-signing-ios-14-6-and-ipados-14-6-downgrade-no-longer-possible/

Apple is no longer signing iOS 14.6 and iPadOS 14.6. You can still download and install updates 14.7 and 14.7.1 for iPhone and iPad.

Having released iOS 14.7.1 and iPadOS 14.7.1 (alongside macOS 11.5.1) with extremely important security fixes, it was only a matter of time the company would stop signing older firmware. Apple does this so that everyone get the benefit of latest software.