I will try to make this as clear as I can for all future readers:
1. If you did not take advantage of the one-year free upgrade to Windows 10 offer before the end of July 2016 for a specific piece of equipment, you must now purchase a product key if you wish to activate Windows 10 on a piece of equipment on which you are installing it.
2. If you did take advantage of that offer, but reinstalled an earlier version of Windows or an entirely different OS on the piece of equipment that had been upgraded to Windows 10, a digital entitlement exists linked to the motherboard of that piece of equipment that will allow you to reinstall Windows 10 and have it activate automatically under that entitlement.
3. If you purchased a machine with Windows 10 on it as its original OS, and installed another OS after the fact, a digital entitlement still exists for that machine and can be used to reinstall Windows 10 and have it automatically activate.
Windows 7 and Windows 8/8.1 product keys will not activate Windows 10 now that the free upgrade period is almost one year past its expiration date.
Those who use assistive technology still qualify for a free upgrade to Windows 10 under the terms set forth by Microsoft. Others are expected to purchase a license key to either install Windows 10 from scratch on a machine that's never had an OS or to update a machine with any pre-existing OS, that has never in its history had the Windows 10 OS on it, to Windows 10.
Personally, I have had very inconsistent results with bootable USB media, and particularly ones created directly by the Microsoft Media Creation Tool. I would far rather download the ISO and either create a bootable USB using something like Rufus or burn it to optical media. When a device has an optical drive I always opt for optical media if the drive is functioning.