There are 2 cases can cause this problem:
Case 1. The most possible reason is that you enabled Find My iPhone on your iPad or iPhone, you can temporary disable it then try to restore again, and re-enable it after restore completed. To turn off Find My iPhone on an iOS device, tap Settings > iCloud, then tap to turn off Find My iPhone.
Case 2. Cause of some security software on your computer, please read this article from Apple for more detail:
iTunes Restore Error Code 37
If you try the method in case 1 and case 2 and the problem still not resolved, you can try restore your device with iTunes on a different known-good computer. If the errors persist on another computer, the device may need service, just take it to Apple Store.
Special Thanks to Don Hudson!
Tags: Restore Error