Sum fields in child table and show total in parent field

ERPNext
2

Hi @msiam,

Please apply custom/client script.

frappe.ui.form.on('Sales Invoice',  {
    validate: function(frm) {
        ttl_itms_dicnt = 0;
        $.each(frm.doc.items,  function(i,  d) {
            ttl_itms_dicnt += flt(d.discount_amount);
        });
        frm.doc.total_items_discount = ttl_itms_dicnt;
    } 
});

Please check your field name and set it according.
Then reload and check it.

Thank You!