I think you are seeing the same error as:
https://www.mobileread.com/forums/sh...9&postcount=21
The solution is that I think something is missing from the LD_LIBRARY_PATH setting that causes a conflict between PySide6 and Sigil that prevents PySide6 from finding the pieces it needs.
DiapDealer will probably know more. See his followups in that thread.
I build my own PySide6 on my Manjaro Linux box and have never seen this error.