Here I already created Serial No field, but it should populate automatically as sequence number whenever the user enters the new line items or for already existed line items. Still after running the script nothing is showing in the execution log. Why ?

function sublistChangedSetSrNo(scriptContext) {
            try {
                //alert(scriptContext)
                var currentRecord = scriptContext.currentRecord;
                //get the line count of item sublist
                var  lineCount = currentRecord.getLineCount({
                    sublistId: 'item'
                });
                log.debug('lineCount',lineCount)

                // we should do loop through line and set the value of the serial.no field to the line sequence no.
                for (var i = 0; i < lineCount; i++){
                log.debug('line',i)
                currentRecord.selectLine({sublistId: 'item',line: i});
                    // set the value of the serial.no field to the line sequence no.
                    currentRecord.setCurrentSublistValue({
                        sublistId: 'item',
                        fieldId: 'custcolcol_serial_no',
                        value: i + 1
                    });
                    currentRecord.commitLine({
                        sublistId: 'item'
                    });
                }
            } catch (e) {
                log.error({
                            title: "error entry",
                            details: e
                        });
            }
        }
    return {
        validateLine: validateLine,
        sublistChanged: sublistChangedSetSrNo
    };