Requirement of the client is to print the packing slip in the invoice record. For that a custom button have to be added in the invoice record. Then pass the suitelet id in clientscript.
UserEventScript to create a custom button in the invoice record to print the packing slip.
/**
 * @NApiVersion 2.1
 * @NScriptType UserEventScript
 */
define([],
    () => {
        /**
         * Defines the function definition that is executed before record is loaded.
         * @param {Object} scriptContext
         * @param {Record} scriptContext.newRecord - New record
         * @param {string} scriptContext.type - Trigger type; use values from the context.UserEventType enum
         * @param {Form} scriptContext.form - Current form
         * @param {ServletRequest} scriptContext.request - HTTP request information sent from the browser for a client action only.
         * @since 2015.2
         */
        const beforeLoad = (scriptContext) => {
            try{
                if(scriptContext.type === scriptContext.UserEventType.VIEW ) {
                    let itemFulfillemnt = scriptContext.newRecord.getValue({
                        fieldId: 'custbody_jj_item_fulfillment_fl_37'
                    });
                        scriptContext.form.addButton({
                            id: 'custpage_packing_slip_button',
                            label: 'Print Bunnings Packing Slip',
                            functionName: `printPackingSlip(${itemFulfillemnt})`
                        });
                    scriptContext.form.clientScriptModulePath = 'SuiteScripts/Jobin and Jismi IT Services LLP/Packing Slip Customization FL-34/FL-37 Print Bunnings Packing Slip/JJ CS Print Bunnings Packing Slip FL-37.js' ;
                }
            }catch(error){
                log.error("Error @beforeLoad",error);
            }
        }
        return {beforeLoad}
    });
Clientscript to passing the script id of the suitelet
/**
 * @NApiVersion 2.x
 * @NScriptType ClientScript
 * @NModuleScope SameAccount
 */
define(['N/currentRecord','N/url','N/log'],
    /**
     * @param{currentRecord} currentRecord
     * @param{url} url
     * @param{log} log
     */
    function(currentRecord,url,log) {
    /**
     * Function to be executed after page is initialized.
     *
     * @param {Object} scriptContext
     * @param {Record} scriptContext.currentRecord - Current form record
     * @param {string} scriptContext.mode - The mode in which the record is being accessed (create, copy, or edit)
     *
     * @since 2015.2
     */
    function pageInit(scriptContext) {
        console.log("jhgwgdkwhd");
    }
    /**
     * function for getting   itemfulfillmentid
     * @param {Object} scriptContext
     * @param {Record} scriptContext.currentRecord - Current form record
     * @param {currentRecord} currentRecord
     *
     */
    function printPackingSlip(itemFulfillment){
        try {
            console.log("itemfulfillment",itemFulfillment)
            var createLinkUrl = url.resolveScript({
                scriptId: 'customscript_jj_sl_print_packing_slip',
                deploymentId: 'customdeploy_jj_sl_print_packing_slip',
                returnExternalUrl: false,
                params: {
                    itemFulfillment: itemFulfillment
                }
            });
            console.log("createLinkUrl",createLinkUrl)
            window.open(createLinkUrl)
        }catch (e) {
            log.debug(e)
        }
    }
    return {
        pageInit: pageInit,
        printPackingSlip: printPackingSlip
    };
});
Suite let to print the pdf
/**
 * @NApiVersion 2.1
 * @NScriptType Suitelet
 */
define(['N/log','N/render'],
    (log,render) => {
        /**
         * Defines the Suitelet script trigger point.
         * @param {Object} scriptContext
         * @param {ServerRequest} scriptContext.request - Incoming request
         * @param {ServerResponse} scriptContext.response - Suitelet response
         * @since 2015.2
         */
        const onRequest = (scriptContext) => {
            try {
                log.debug("On request","qwjfhwilfhi3");
                if (scriptContext.request.method === 'GET') {
                    const record = scriptContext.request.parameters.itemFulfillment;
                    const printInvoice = render.transaction({
                        entityId: parseInt(record),
                        formId: 191,
                        printMode: render.PrintMode.PDF,
                        inCustLocale: true
                    });
                    scriptContext.response.writeFile(printInvoice,true)
                }
            } catch (error) {
                log.error('Error @onRequest', error);
            }
        }
        return {onRequest}
    });