NTUSER.ini still exists even in Windows 10, however I'm not sure it actually contains valuable information anymore. I opened a few in notepad they are about 20 bytes and don't seem to really contain much of anything just a few bytes of binary. It used to be used to set different settings on your profile especially when it came to user roaming. Where are you getting an error about the file missing? Part of Windows 10 upgrade asks you if you want to keep anything, if your prepping to sell it I would just tell it to keep nothing and perhaps that will avoid the error as you will have to make a new user anyways?