Hello Khadhraoui,

Please try below code :-

<record id="view_oppor_form" model="ir.ui.view">
    <field name="name">crm.oppor.form</field>
    <field name="model">crm.lead</field>
    <field name="inherit_id" ref="crm.crm_case_form_view_oppor/>
    <field name="arch" type="xml">
        <xpath expr="/form/sheet/notebook/page[@string='Internel notes']" position="after">
            <page string="Page String" attrs="{'invisible' : [('stage_id.name', '=', 'lost')]}">
                // code of page
            </page>
        </xpath>
    </field> 
 </record>

It means when stage is lost, this page which you add is invisible.

Hope it will helps you.

Thanks,