fix(packing-slip): add a descriptive message (#18981)

diff --git a/erpnext/stock/doctype/packing_slip/packing_slip.js b/erpnext/stock/doctype/packing_slip/packing_slip.js
index 9f6b9ff..bd14e5f 100644
--- a/erpnext/stock/doctype/packing_slip/packing_slip.js
+++ b/erpnext/stock/doctype/packing_slip/packing_slip.js
@@ -10,7 +10,7 @@
 
 cur_frm.fields_dict['items'].grid.get_field('item_code').get_query = function(doc, cdt, cdn) {
 	if(!doc.delivery_note) {
-		frappe.throw(__("Please Delivery Note first"))
+		frappe.throw(__("Please select a Delivery Note"));
 	} else {
 		return {
 			query: "erpnext.stock.doctype.packing_slip.packing_slip.item_details",
@@ -125,4 +125,4 @@
 	return '<table style="width:100%">' + make_row('Gross Weight', doc.gross_weight_pkg) + '</table>'
 }
 
-// TODO: validate gross weight field
\ No newline at end of file
+// TODO: validate gross weight field