Hi, I'm Elise, and I'd be happy to help with your issue.
Have you made any hardware changes to your PC, often Windows will not be able to match the OEM key to the hardware if this has been done.
There are a few steps you can try take to resolve this which you can find on this article:
https://support.microsoft.com/en-us/windows/get...
Please let me know if you need any further assistance.
Kind Regards,
Elise