Microsoft is aware of the issue and is now working on a fix. This included both Windows Insider testers and those builds released to mainstream users (including some users with the Windows 10 October 2018 update/1809).
Some Windows 10 users are reporting license issues resulting in warnings that their system is not activated.
The cause for this is being attributed to the failure of Microsoft's Activation Servers to validate the Windows 10 licenses.
A subsequent fix for the 1809 build resulted in some HP systems suffering from the Blue Screen of Death, which triggered another update to fix issues with driver compatibility. The company issued an Emerging Issue notice, and a copy of a technical support chat states that the activation error is known to impact Windows 10 Pro users in the US, Japan, Korea, and other unnamed countries.
The agent said some Windows users might be receiving messages saying Windows is not activated and that this was due to "a temporary issue with Microsoft's activation server", which would be fixed in "one to two business days".
If you are facing the issue, you can resolve it using the Activation troubleshooter, located in the Windows settings, under the Update and Security.
You can try to click on "Troubleshoot" link and Windows will try to detect any problems.
Twitter user Matt Wadley was one of the first out of the gate, complaining that following an update to the freshly released Insider build of next year's Windows, his machine now thought it had a Windows 10 Home licence. The app is only available in the US, UK, and Germany, but Amazon plans to launch it in more locations next year.
And you're not alone in this annoyance.
The vast majority of issues reported so far appear to be from users who upgraded from a previous version. It's not yet known what is causing the problem but rest assured that Microsoft is well aware of the problem and is working to get everything running smoothly again.