To be sure that you'll be able to reinstall Windows 10 at some future time, without buying a copy, tap on the following link and follow the procedure now.
Tap or click here
Also, before starting, set "sleep" to "never". Then, when done, set back to your specified time. Otherwise, the long process may not be able to finish and you will get a message saying that the drive could not be created.
If you must use battery, be fully charged and turn off the display,
You would think the app would have attended to the above.