Unable to find validation rules when using $this->form_validation->set_rules

I can't figure out why I'm getting the error "Unable to find validation rules" when I set the rules in the controller using $this->form_validation->set_rules.
Here is my JS code: 

Code:

            var id = $('#pkStudyYears').val();
            var data = {};
           
            data['pkStudyYears'] = id;
            data['years']={};
            data['years']['pkStudyYears']=id;
            data['years']['fkStudyID'] = $('#fkStudyID').val(); 
            data['action'] = action;

            data['years']['intStartYr']= $('#startYearBox').val();
            data['years']['intEndYr']= $('#endYearBox').val();
           
            var posturl = '<?php echo site_url('manage/climate_indicators/updateStudyYears');?>'
         
    $.when($.ajax({type: "POST", url: posturl, data:data, dataType: 'json'})).done(function(result) {
                console.log(result.status);
});

And php:

PHP Code:

  public function updateStudyYears() {
        
        if (!$this->input->post() OR !$this->input->is_ajax_request() ){
            echo json_encode(array('status'=>'failed','errors'=>array('Access Mode'=>'Disallowed')));
            exit();
        }
        else {
            $postData = $this->input->post(NULL, true);
            
            $yearArray = $postData['years'];
            
            if(empty($yearArray)) {
                echo json_encode(array('status'=>'failed', 'errors'=>'Years are empty'));
                exit();
            }    
            $this->form_validation->set_rules('years[intStartYr]', 'Start Year', 'required',
                    array('required' => 'Start year must have a valid year.'));
            $this->form_validation->set_rules('years[intEndYr]', 'End Year', 'required',
                    array('required' => 'End year must have a valid year.'));
            
            if ($this->form_validation->run() === FALSE) {  
                $validationErrs = $this->form_validation->error_array();
            
                echo json_encode(array('status'=>'form-errors', 'valErrs'=>$validationErrs));                
                exit;
            }
            else {                        
                $status = $this->climate_indicators_model->setStudyYears($yearArray);            
                if ($status) {
                    echo json_encode(array('status'=>'success', 'message'=>'Study year information successfully updated.'));                    
                }
                else echo json_encode(array('status'=>'failed', 'errors'=>'Db error on update'));
            }
            exit();
        }
    }    


Thank you!