odoo16/addons/l10n_in_sale/views/report_sale_order.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>

    <template id="gst_report_saleorder_document_inherit" inherit_id="sale.report_saleorder_document">
        <xpath expr="//span[@t-field='line.name']" position="after">
            <t t-if="line.product_id.l10n_in_hsn_code"><h6><strong class="ml16">HSN/SAC Code:</strong> <span t-field="line.product_id.l10n_in_hsn_code"/></h6></t>
        </xpath>
    </template>

</odoo>