To eliminate the error, replace the below-mentioned coding with the correct one.

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

<odoo>

    <data>

        <record id="purchase_paper_format" model="report.paperformat">

            <field name="name">Purchase Order</field>

            <field name="default" eval="False"/>

            <field name="format">custom</field>

            <field name="page_height">297</field>

            <field name="page_width">210</field>

            <field name="orientation">Portrait</field>

            <field name="margin_top">3</field>

            <field name="margin_left">15</field>

            <field name="margin_right">15</field>

            <field name="header_line" eval="False"/>

            <field name="header_spacing">3</field>

            <field name="dpi">120</field>

        </record>

Replace:

        <record id="purchase_format" model="ir.actions.report">

            <field name="paperformat_id" ref="report_purchase_order.purchase_paper_format"/>

        </record>

    </data>

</odoo>

With:

<record id="purchase.action_report_purchase_order" model="ir.actions.report"> <field name="paperformat_id" ref="report_purchase_order.purchase_paper_format"/> </record>