Hi,
There could be an issue with the Upgrade Advisor app. The Lumia 635 1GB supports the Windows 10. We advise you uninstall the Upgrade Advisor app, then perform a soft reset to refresh the phone's software. To soft reset, just press and hold the power and volume down buttons at the same time until your phone restarts.
Once the phone is up and running, download and install the Upgrade Advisor app, then try to upgrade the phone's software to Windows 10 again.
Please let us know what happened.
Regards.