In the configuration menu, you must set the pathes of Inskape and blender like /usr/bin/inskape if inskape is under /usr/bin
I'm having the same issue. I do not have Inkscape installed, so my question is, do you have to install it in order to change the font size?
Question has been answered by pastjl.
So the answer to "Do you have to install Inkscape in order to change the font size" is "In the configuration menu, you must set the paths of Inkscape, etc.". I guess I'll take that as a "yes".
But I would suggest that this is a flaw in the design. Why give the user controls to change the font and size, when even after configuring the Inkscape path, you still need to launch it via "advanced" in order for it to work?