So it's a bit CONFUSING, though it looks like W10 is suggested.
If you just bought W8.1 recently, I suppose you could contact Microsoft Support and see if your "mistake due to MS/Kaby incompatibility" could allow for a W10 free update. Microsoft is pretty weird with what is allowed and what is not.
There's also the free ASSISTIVE version of Windows 10 https://www.howtogeek.com/265409/you-can-still-get-windows-10-for-free-from-microsofts-accessibility-site/
I don't know the EXACT method for that though. If that works, may want to look into selling your W8.1 code.
Big HASSLE I know. Maybe someone else has better advice.