account-financial-tools/account_fiscal_position_vat_check/views/res_partner.xml

<?xml version="1.0" encoding="utf-8" ?>
<!--
  Copyright 2022 Akretion France (http://www.akretion.com/)
  @author: Alexis de Lattre <alexis.delattre@akretion.com>
  License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl).
-->
<odoo>

    <record id="view_partner_property_form" model="ir.ui.view">
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="account.view_partner_property_form" />
        <field name="arch" type="xml">
            <div role="alert" position="after">
                <div
                    class="alert alert-warning"
                    role="alert"
                    groups="account.group_account_invoice,account.group_account_readonly"
                    attrs="{'invisible': [('show_warning_vat_required', '=', False)]}"
                >
                    <b>Missing VAT number</b>: this partner has the fiscal position <em
                    ><field
                            name="property_account_position_id"
                            readonly="1"
                        /></em> that require to know the VAT number of the partner.
                </div>
            </div>
            <group name="sale" position="inside">
                <field name="show_warning_vat_required" invisible="1" />
            </group>
        </field>
    </record>

</odoo>