In Purchase Order, there is Deliveries & Invoices tab I have to apply the groups to "purchase.group_purchase_manager"

but there is no any effect. so how to apply groups into this Page

I try this code of the apply group 

  <xpath expr = "//page[2]" position = 'attributes'>    

        <attribute name = "groups"> purchase.group_purchase_manager </ attribute> 

    </xpath> 

also try:~

    <xpath expr = "//page['Deliveries &amp; Invoices']" position = 'attributes'>    

        <attribute name = "groups"> purchase.group_purchase_manager </ attribute> 

    </xpath>

but not working for me.