This seems to be a bit of a mystery. Microsoft uses the initials MUI for its "Multilingual User Interface Technology", but clearly that is not what this registry key is for.

There's some information on the MUICache here;

Follow it through, and you will see a fine description of it by the poster "inbfedsp", about 2/3rds of the way down.

As to your problem I am not sure. The MUICache entry should, it seems to me, be made every time you open that Poivy application, but as it is not doing so, I wonder if the application has faulted, or become corrupt in some way. Have you re-booted the computer and tried again? It may be a temporary glitch.

If not, I would suggest you uninstall the application, re-boot, (restart), the computer to update the registry, then try re-installing the application again.

Sadly it seems that Poivy does not have any FAQs or a Forum, where you might have been able to find out more, but they do have a contact option under their Feedback section;

Good luck.