ERROR_SXS_ASSEMBLY_MISSING

Had an issue recently trying to add the IIS feature on a Windows 2019 server. Adding it would fail with error 0x80073701. C:\Windows\Logs\CBS\CBS.log had the following:

CBS Failed to pin deployment while resolving Update: Package_2949_for_KB4538461~31bf3856ad364e35~amd64~~10.0.1.7.4538461-4917_neutral from file: (null) [HRESULT = 0x80073701 - ERROR_SXS_ASSEMBLY_MISSING]

Google turned up some good articles like the one below:

https://thatonecomputerguy.wordpress.com/2015/10/06/windows-update-fails-with-error-code-0x80073701-error_sxs_assembly_missing-solved/

Cool. So I’ll just download that update from the Windows update catalog:

https://www.catalog.update.microsoft.com/Search.aspx?q=KB4538461

Sad face.

What I ended up doing was running setup.exe from a Windows 2019 installer ISO and basically doing an in place upgrade to the same version. After the “upgrade” I was able to add the feature without issue.

I probably also could have deleted all the reg entries that referenced the bad update (the update couldn’t be uninstalled because Windows didn’t see the update as installed in the first place), but there were over 8,000 references to the update so it wasn’t practical in this case.

Hope this helps someone else.