That is a problem with the hubstats, as the code is looking for the function in the hubstats.class.php file, but not finding it. I did not create that file, Tom Sterritt did, so he is the person to have to go to to get support for his file. You can try downloading it from his github page. The link is in the readme file. Also the hub must be already listed as a hub in the airports database. If you just put in an airport, it will give out errors. So go into your airports section of the admin panel, and click on the airport, and chech the hub checkbox, and it should work fine then.