Plugin is throwin an error and not working in game

The answer to the problem is actually in your post,
Check you have a Stacksize data file (which is now not used for adjusting amounts) if you dont then delete plugin (save your CONFIG file so you can adjust new install) then reinstall plugin , Then check it has downloaded defaults from github for data file (which should happen each time your server starts) , readjust CONFIG settings and reload to save those settings