Why you can’t find your product key after upgrading to Windows 10
Entitlements, not product keys
When you use the free upgrade to Windows 10 or pay for a digital retail version, you don’t get a product key. Instead, your computer uses what Microsoft calls a digital entitlement.
“Upon upgrading to Windows 10, the activation state from a prior version of Windows (be it Windows 7, Windows 8 or a prior build of Windows 10) is seamlessly carried over,” a Microsoft spokesperson told PCWorld. “Once activated, a digital entitlement for your PC’s hardware is created in the Windows 10 activation service.This entitlement can be used by the same PC again for re-activation of the same Edition of Windows 10 in the future.”
It looks like the product key is on its way out, at least as far as users are concerned. After installation, your version of Windows 10 should just activate in the background in a few moments or a few days at the most. You can check your activation status by going to Settings > Update & Security > Activation.