The software update to upgrade to Windows 11 published to WSUS may not be returned as applicable to Windows 10 devices for several reasons. Devices running Windows 10, version 1909 should be patched up to October 12, 2021 (KB5006667) or later. Devices running Windows 10, version 2004+ should be patched up to September 14, 2021 (KB5005565) or later. In addition to the minimum patch level requirements being met, devices will also need to meet the Windows 11 hardware requirements before the Windows 11 upgrade is returned as applicable.
NOTE: There are also requirements for virtual machines (and their hosts) that must be met before VMs return the Windows 11 upgrade as applicable.
Microsoft recommends leveraging the Work from anywhere (preview) report in Endpoint analytics - Microsoft Endpoint Manager as a scalable solution to determine Windows 11 readiness and help determine why Windows 10 devices are not returning the Upgrade to Windows 11 software update as applicable/required in Configuration Manager.