it usually means the settings.json file is missing or corrupted. Getting a new copy of it should fix the problem
I haven't been able to test whether this is an issue with Mac
Viewing post in Portrait Pack Editor Redux (Not a Mod)
that's the default value. it tells the editor to find the portraits automatically
if changing the value to your portrait's actual directory helps, then the issue is with how the editor works in Mac
first try changing the PackDirectory to "C:\\"
the editor should still run even if the folder is incorrect