I just installed this plugin as a test and it didn’t give me any errors. When you go to the admin panel for its “icons” section, can you select an icon for a category? It seems that it shouldn’t even need to write to that directory, as you need to upload the icons as a WordPress media file like any other gallery picture.
It seems to me that the default directory is more likely non-existant and needs to be properly set. Are the Options fields for Local Path and URL blank? If I put a bogus Local Path, I get that error as shown in the image below: