Just adding to Holdum's comment; Microsoft only allows 31 days to perform the W10-->Win7 reversion. So, if you did your W10 upgrade more than 31 days ago (< March 10), this will not work. Microsoft did this intentionally to let people try the W10 product out for a few weeks and made an easy reversion tool to let people go back. However, they did not advertise this time limit well, and we W10 Insider Testers (about 5 million of us) lobbied MS hard to do so, but they didn't do a good job here with the dissemination of this vital piece of information.
If the reversion fails, you can always take it to your local computer repair shop that uses licensed Techs (A+ or better) and pay them to put back to Win7 using recovery tools at their disposal.