OK, I have removed the SNAP inkscape and installed Inkscape 9.2 with apt-get. Now I get a different message when trying to plot from the Axidraw extensions:
AxiDraw software must be run with python 3.5 or greater.
I am running the Anaconda distribution of Python 3.7 :
(base) glenn@XPS-8100:~/Desktop$ python
Python 3.7.6 (default, Jan 8 2020, 19:59:22)
[GCC 7.3.0] :: Anaconda, Inc. on linux
My PYTHONPATH is /home/glenn/anaconda3