MUI is the Windows "Multilingual User Interface", I'm not sure just how much it is implemented in Win 11 as of yet so wouldn't regard missing references as a problem. (Unless you are using a language other than English).
However I wouldn't be using the Registry Cleaner, any registry cleaner, with Windows 11 (or with Windows 10 come to that).
With both Win 10 and 11 the registry changes too often for it to be safe to regularly run a registry cleaner on, you run the risk of removing something needed, or even bricking your machine and needing to reinstall Windows from scratch.
See this: <a href="<___base_url___>/topic/59952-i-get-a-registry-error-on-ccleaner-on-windows-10-i-have-scanned-5-times/?tab=comments#comment-326804" ipsnoembed="true" rel="">https://community.ccleaner.com/topic/59952-i-get-a-registry-error-on-ccleaner-on-windows-10-i-have-scanned-5-times/?tab=comments#comment-326804</a>