iTunes not recognized my i pad
error 1668, what to do
Checking this support document, the error does not appear, but the number is close to ones that indicate a USB error. Get help with iOS update and restore errors - Apple Support
Try the information in that support document for USB errors. If you are using a Windows computer, you might want to take a look at this support document as well. iPhone, iPad, or iPod not recognized in iTunes for Windows - Apple Support
It may require you to remove and reinstall iTunes. The link for instructions to remove are in that support document under the Windows 8 section. They need to be followed exactly to ensure it works correctly.