I suspect you have installed the snap (https://tutorials.ubuntu.com/tutorial/basic-snap-usage) version of Inkscape. Snap is an extra safe, and thus extra restrictive way of installing applications on Ubuntu/Linux.
In my view, that package format still very much a 'work-in-progress', and there already exist better ones (others may disagree).
To check, do
snap list
on the command line.
Uninstall the snap version on the command line doing:
sudo snap remove inkscape
then install the 'real' Inkscape program from the ppa, following the instructions here:
https://launchpad.net/~inkscape.dev/+archive/ubuntu/stable