Codeigniter where not exists does not work

I'm working on Codeigniter project and came across a problem. I would like that my function on my model to bring me the data from the POST table, where POST_ID in CATEGORY Table is not null, without joining the table, so (do not bring back the data whose POST_ID in CATEGORY table not exists). so i tried this method how can i get this result?

Code:

code: // MyModel

function get_data() {
   
   
    $id = isset($_REQUEST['id']) ? $_REQUEST['id'] : '';

    if ($id != null) {
        $this->db->where('ID',$id);
        $result =  $this->db->select('POST.ID,POST.DATE,'
        . ' TYPE.Post_Type P_TYPE,)
        ->join('TYPE', 'POST.TYPE_ID = TYPE.ID', 'left')
        ->order_by('POST.ID', 'DESC')
        ->where_in('POST.Post_Type  = type_1, type_2')
        ->where("NOT EXISTS(SELECT POST_ID FROM CATEGORY WHERE POST.ID=CATEGORY.POST_ID )", FALSE)
        ->get('POST');
    }
    return $result;
}

Thank you for help