Update to iOS 9.3.2 results in bricked 9.7-inch Apple iPad Pro tablets
Apple's support document says that Error 56 is a hardware issue, and the document suggests that the user install the latest version of iTunes, and check to make sure that there are no problems with third-party security software. It recommends checking the USB cable. the tablet and the network to make sure that they are all reliable, whatever that means. You should then try to restore the iPad Pro twice. If the error message continues to appear, contact Apple support.
In a situation like this, where there is no quick fix that is working, it will be up to Apple to disseminate a cure in the form of a new software update. Apple will no doubt try to get this out ASAP, especially since there are a number of iPad Pro owners who can't use their slates until the issue is all cleared up.
source: AppleSupport via MacRumors