You are missing pieces. You need to install pyqt5.
Here is a link I found by a websearch that explains how to get the qtwebengine piece of pyqt5. And something similar should work on the main pyqt5 itself.
https://installati.one/ubuntu/22.04/...5.qtwebengine/
There are probably better links but that was the first I found.
Update:
Try the following:
sudo apt-get install python3-pyqt5
sudo apt-get install python3-pyqt5.qtwebengine
Both macOS and Windows builds of Sigil come with these installed. I truly have no idea why various Linux distributions treat these packages as "optional" when full plugin support is part of Sigil, has been for a long time, and will continue to be.