Apple has stopped signing iOS 14.5.1 following the release of iOS 14.6 a couple of weeks ago. That means that, if you’ve already upgraded to the latest version of Apple’s mobile operating system, there’s no going back again. (Although, to be fair, why would you want to?)
Apple stops signing firmware quickly after introducing newer, more up-to-date versions. “Signing” is the process of confirming that code is legitimate and has not been altered or corrupted. This allows downloaders to know their software updates are coming through official Apple channels.