Hi @DIVI1 ,

Try below code 

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

    var ticketcmdb = 'GFP 0020764392 000';
    var ticketcat = 'Human Resources Payroll';

    var r = new sn_ws.RESTMessageV2('NTT API Integration', 'NTT POST');
    r.setStringParameterNoEscape('u_description', current.description);
    r.setStringParameterNoEscape('u_short_description', current.short_description);
    r.setStringParameterNoEscape('u_state', current.state.getDisplayValue());
    r.setStringParameterNoEscape('u_priority', current.priority);
    r.setStringParameterNoEscape('u_system', ticketcmdb);
    r.setStringParameterNoEscape('u_category', ticketcat);
    r.setStringParameterNoEscape('u_number', current.number);
    r.setStringParameterNoEscape('u_comments', current.comments);
    r.setStringParameterNoEscape('u_work_notes', current.work_notes);
    r.setStringParameterNoEscape('u_assignment_group', 'SAP');
    r.setStringParameterNoEscape('u_correlation_number', current.isp_ticket);

    var response = r.execute();
    var responseBody = response.getBody();
    var statusocde = response.getStatusCode();
    var errormsg = response.getErrorMessage();

    gs.info("responseBody" + responseBody);

    var responseObj = JSON.parse(responseBody);
    gs.info("Correlation number check " + responseObj.correlation_number);
    current.u_isp_ticket = responseObj.correlation_number;
    current.work_notes = "information: Ticket successful created. Ticket number: " + responseObj.correlation_number;
    current.update();

})(current, previous);

Still if you are facing share the logs and check what values you are receiving