Bug was reported to KDE upstream, which they then said to bring to you. Was
Okular not compiled with ghostscript support?

https://bugs.kde.org/show_bug.cgi?id=426064

I am copying the report here in verbatim:

> SUMMARY
> 
> Okular is not loading print previews.
> 
> STEPS TO REPRODUCE
> 1. Open Okular
> 2. Load any PDF
> 3. File>Print Preview
> 
> OBSERVED RESULT
> 
> A window which says "could not load print preview part".
> 
> EXPECTED RESULT
> 
> A print preview of the PDF, based on my printer page layout.
> 
> SOFTWARE/OS VERSIONS
> 
> Linux/KDE Plasma: openSUSE tumbleweed 20200826 snapshot (Linux <host>
> 5.8.2-1-default (71b519a) x86_64 GNU/Linux)
> KDE Plasma Version: 5.19.4
> KDE Frameworks Version: 5.73.0
> Qt Version: 5.15
> okular Version: 1.11.0
> 
> ADDITIONAL INFORMATION
> 
> Opening okular from command line provided me a hint: org.kde.okular.ui:
> Could not find a PS viewer for the preview dialog.
> 
> When I tried to load a print preview for a .odt file, the preview loaded
> successfully. I don't have files in many different supported formats so I
> have not checked all of them.