https://wccftech.com/apple-has-stopped-signing-ios-14-5-and-ipados-14-5-downgrade-from-14-5-1-no-longer-possible/

Apple has just stopped signing iOS 14.5 and iPadOS 14.5 firmware for iPhone and iPad. This means you can no longer downgrade from iOS 14.5.1 and iPadOS 14.5.1.

As expected, the iPhone maker has stopped signing iOS 14.5 and iPadOS 14.5. In other words, if you were planning to downgrade from iOS 14.5.1 or iPadOS 14.5.1, you can no longer do that. Instead, you can re-restore your device to iOS 14.5.1 and iPadOS 14.5.1 if you are having issues.