the error is from the Group && Staff plugin which has a different database format, in fact if you look at the database you will see that it does not create any for you, when it should create $ wcfm_shop_staffs_table = '';
therefore in the front when it tries to make the call to the data it returns the error that that id has no data.
Code:
DataTables warning: table id=wcfm-shop-staffs - Invalid JSON response. For more information about this error, please see http://datatables.net/tn/1
in my case, it is not important since I do not need to add more stafs or groups