Following the release of iOS 11.0.1 and iOS 11.0.2 on September 26 and October 3, respectively, Apple has stopped signing both iOS 10.3.3 and iOS 11.0, the previous versions of iOS that were available to consumers. 

iPhone, iPad, and iPod touch owners who have upgraded to iOS 11.0.1, iOS 11.0.2, or iOS 11 will no longer be able to downgrade to the iOS 10.3.3 operating system. 

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 11.0.1 and iOS 11.0.2 are now the only versions of iOS 11 that can be installed on iOS devices by the general public, but developers can download iOS 11.1, a future update that is being beta tested and will be released in the near future.

Source: macrumors