It would appear you've installed the snap version of inkscape which has no ability to use the removeable-media plugin so you're restricted to only opening files in $HOME. To check run this
If so either continue as you're presently doing if wanting to use the snap version, use the snap edge version or install the .deb version which has no restrictions.
The edge (dev) snap can be connected to removeable media though isn't by default. Maybe at some point the stable version will also get that possibility..
To do that,
sudo snap refresh inkscape --edge
Then connect to plugin
snap connect inkscape:removable-media
For the .deb version just,
sudo apt-get install inkscape
You can keep both versions or remove the snap. The snap version is shown as Inkscape, the .deb as Inkscape Vector Graphics Editor though from context menu both are just inkscape..