It sounds to me as if the lxml python3 module isn't installed. Commenting out imports from Sigil's custom bs4 module is not going to "fix" the problem. It will just move it. The fix is to make sure all required python3 modules are installed:
Code:
sudo apt-get install python3-lxml python3-six
In order to make full use of the available 3rd-party plugins for Sigil, you'll also want to make sure the following
recommended python3 modules are installed.
Code:
sudo apt-get install python3-tk python3-pyqt5 python3-html5lib python3-regex python3-pillow python3-cssselect python3-cssutils python3-chardet