AJAX Call – Failed to load resource: the server responded with a status of 500 (Internal Server Error)

I am calling get_cat() function from update_passage_list() function

This is what you’re doing:

$cpid = get_cat($cpid);

There’s nothing there that says that get_cat() is a method of your class. It’s trying to find a function get_cat() outside any class. To use the method of the class you need to use $this:

$cpid = $this->get_cat($cpid);