NOTE: The "ref_lot_serial_number" field is added in stock_move

models/traceability_report.py

from odoo import models, api

class MrpStockReport(models.TransientModel):
    _inherit = 'stock.traceability.report'

    def _make_dict_move(self, level, parent_id, move_line, unfoldable=False):

        result = super()._make_dict_move(level, parent_id, move_line, unfoldable)

        ref_lot_serial = move_line.move_id.ref_lot_serial_number if move_line.move_id.ref_lot_serial_number else ''

        print(f"level {level} parent_id {parent_id} move_line {move_line} ref_lot_serial {ref_lot_serial}")

        for rec in result:
            rec['ref_lot_serial_number'] = ref_lot_serial

        print(f"result {result}")
        return result

    @api.model
    def _final_vals_to_lines(self, final_vals, level):

        lines = super()._final_vals_to_lines(final_vals, level)

        for line in lines:
            line['ref_lot_serial_number'] = ''

            matching = next((f for f in final_vals if f['model_id'] == line['model_id']), None)
            if matching and 'ref_lot_serial_number' in matching:
                line['ref_lot_serial_number'] = matching['ref_lot_serial_number']

                line['columns'].insert(4,line['ref_lot_serial_number'])

        return lines

views/traceability_report.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <template id="inherit_report_stock_inventory" inherit_id="stock.report_stock_inventory">
        <xpath expr="//thead/tr[@class='o_report_header']/th[contains(., 'Lot/Serial #')]" position="after">
            <th class="o_report_line_header">Ref Lot/Serial</th>
        </xpath>
    </template>

    <template id="inherit_report_stock_body_print" inherit_id="stock.report_stock_body_print">
        <xpath expr="//thead/tr[@class='o_report_header']/th[contains(., 'Lot/Serial #')]" position="after">
            <th class="o_report_line_header">Ref Lot/Serial</th>
        </xpath>
    </template>
</odoo>

the result:

Hii,

Here is custom model please try this

Python: Add Custom Field
you will already add

XML: Extend Traceability Report Template

In your custom module (views/report_traceability_line_inherit.xml), use the following code:

<odoo>
  <!-- Inherit the QWeb template used for the traceability report -->
  <template id="report_traceability_line_inherit_ref_lot" inherit_id="stock.report_traceability_line">

    <!-- Add new column header -->
    <xpath expr="//thead/tr" position="inside">
      <th>Reference Lot/Serial</th>
    </xpath>

    <!-- Add new column data -->
    <xpath expr="//tbody/tr" position="inside">
      <td>
        <t t-esc="line.reference_lot_serial or ''"/>
      </td>
    </xpath>

  </template>
 </odoo>

__manifest__.py

Ensure the view is loaded in your manifest:

'data': [
    'views/report_traceability_line_inherit.xml',
],

i hope it is usefull