i want to add a field to the sale.order view 

i proceed this way for the declaration of the field on the model.py 

class SaleOrderLine(models.Model):

    _inherit = 'sale.order.line'

    prf = fields.Char(string='prf')

and on the view form the fallowing :

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

<!-- License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl.html).

     Copyright 2015 Grupo ESOC Ingeniería de Servicios, S.L.U. -->

<odoo>

    <data>

                <record model="ir.ui.view" id="sale_order_view_inherit">

                <field name="name">sale.order.view.inherit1</field>

                <field name="model">sale.order</field>

                <field name="inherit_id" ref="sale.view_order_form"/>

                <field name="arch" type="xml">

                    <xpath expr="//form/sheet/notebook/page[@string='Other Information']/group/group[@string='Sales Information']/field[@name='user_id']" position="after">

                        <field name="prf"/>

                    </xpath>

                </field>

            </record>

    </data>

</odoo>

but the server persiste on showing this error, i think i still have an issue on the xpath structure. don't you think ?