I'm using freecad on Ubuntu 20.04 (the snap version, as the apt version had issues), and it supports the use of some external tools, e.g. graphviz. Those tools usually live in /usr/bin, however due to the nature of snap it can't see my /usr/bin, just the "virtual" one.
How do I make my "regular" /usr/bin available to it?
I tried symlinking it in my home directory and pointing freecad to that, but it still complains it can't find it.
I also had this problem with the distribution version of FreeCAD 19. It does appear that because this is a snap package with its own virtual environment it cannot access 'normal' file system installations such as graphviz.
Running the AppImage instead of the distribution version allowed me to access the Dependency Graph which requires graphviz in FreeCAD.
Perhaps someone knows a way to work around this shortcoming of the Ubuntu distribution of FreeCAD.
Snaps by default are confined and access to filesystem is restricted. But if you really need it, you can always remove confinement by installing snap with --classic option.
So, you need to remove freecad and install it like this:
sudo snap install freecad --classic