The newer version (version 1511 and above) of Windows 10's digital licenses are essential a private key (as in asymmetric cryptography, called digital entitlement, stored in UEFI), as opposed to the a 25-character product key we used before. If for whatever reason the activation program cannot find that key, it won't be activated.
> which in my case was from almost 10 years prior. Luckily enough, I had it archived.
If you bought your Windows license online with a Gmail address, Google has you covered. https://myaccount.google.com/purchases