Hello @Paul Sievers ,
Welcome to Microsoft Q&A forum.
Please make sure that you downloaded and installed the WDK from here: Install WDK. Also, please make sure that, in Visual Studio, you have installed the "Windows Driver Kit" extension(Extensions > Manage Extensions).
If you have installed above two extensions, please try to reinstall WDK.
Open Control Panel > Programs > Programs and Features > find "Windows Driver Kit – Windows 10.0.26100.1" > right-click it > click Uninstall.
Open Visual Studio > Extensions > Manage Extensions > find "Windows Driver Kit" extension > click Uninstall > close VS to uninstall it.
Download WDK from above link which I shared, install the WDK extension into VS again.
Feel free to let us know the results:)
Best Regards,
Tianyu
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