Following the release of iOS 10.3.3 on July 19, Apple has stopped signing iOS 10.3.2, the previous version of iOS that was available to consumers. 

iPhone, iPad, and iPod touch owners who have upgraded to iOS 10.3.3 will no longer be able to downgrade their devices to iOS 10.3.2. 

Apple routinely stops signing older versions of software updates after new releases come out in order to encourage customers to keep their operating systems up to date. 

iOS 10.3.3 is now the only version of iOS 10 that can be installed on iOS devices by the general public, but developers and public beta testers can download iOS 11, a future update that is being beta tested and will be released in the fall.

Source: macrumors