As far as I know, the feature update for version 21H2 has not been released into the WSUS console right now. This version is still in the Preview channel. Please be patient for several days.
In addition, the Windows 10, versions 2004, 20H2, and 21H1 share a common core operating system with an identical set of system files and Windows 10, version 1903 and 1909 share a common core operating system with an identical set of system files. So it is recommended to approve the below Enablement Package to upgrade to windows 10 1909 first and then approve the windows 10 21h1 feature update package for the clients:
Hope the above will be helpful.
Regards,
Rita
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