If you are running your Business Rule on sc_task table and I think you should be able to access the field simple by current.<field_name>;

I can see that you are using current for every table, i don't think that's possible, you need to GlideRecord the table and then assign the value to that particular table's field..

example:

(function executeRule(current, previous /*null when async*/) {

    var sc_req_item = new GlideRecord('sc_req_item');

    sc_req_item.get(parent_id);

    var gr = new GlideRecord('sc_task');

    gr.addQuery('parent', sc_req_item.sys_id);

    gr.addQuery('state', '1').addOrCondition('state', '-5');

    gr.query();

    if (gr.next()) {

          if (sc_req_item.stage.changes())

                sc_req_item.update();

    } else {  

          sc_req_item.stage = 'complete';

          sc_req_item.update();

    }

}