I have a workstation board from around the same time. The issue is probably that Haswell/Broadwell-era processors are no longer supported. (For Intel CPUs, Windows 11 requires 8th generation (Coffee Lake) and higher.)
“Luckily” I already had a physical Windows 2019 Server on my LAN, had Windows 10 Enterprise on all my clients, and had my own Active Domain set up, because that was the only official way to configure it to spy on me less†, so it was easy enough to create an additional Group Policy pinning `ProductVersion` to `Windows 10` and `TargetReleaseVersion` to `1909`‡ and WMI-filtering it to my Haswell Xeon machine so it would stop nagging me: https://learn.microsoft.com/en-us/windows/client-management/...
† I'm still not optimistic enough to say “configure it to not spy on me” lol