https://wccftech.com/apple-is-no-longer-signing-ios-15-0-and-ipados-15-0-for-iphone-and-ipad/

You can no longer downgrade iOS 15.0.1 and iPadOS 15.0.1 back to iOS 15.0 and iPadOS 15.0 on your iPhone and iPad.

The reason for this is simple - Apple has stopped signing the initial firmware release of iOS 15 and iPadOS 15. If you’re already on the firmware, then you have to update to iOS 15.0.1 and iPadOS 15.0.1, eventually. You cannot restore your device to iOS 15.0 or iPadOS 15.0 using Finder or iTunes. It has to be iOS 15.0.1 or iPadOS 15.0.1 at this point.