diff --git a/erpnext/selling/doctype/sales_order/sales_order.py b/erpnext/selling/doctype/sales_order/sales_order.py
index da838d1b795..485ac60e744 100755
--- a/erpnext/selling/doctype/sales_order/sales_order.py
+++ b/erpnext/selling/doctype/sales_order/sales_order.py
@@ -95,18 +95,26 @@ class SalesOrder(SellingController):
 				and customer = %s",
 				(self.po_no, self.name, self.customer),
 			)
-			if (
-				so
-				and so[0][0]
-				and not cint(
+			if so and so[0][0]:
+				if cint(
 					frappe.db.get_single_value("Selling Settings", "allow_against_multiple_purchase_orders")
-				)
-			):
-				frappe.msgprint(
-					_("Warning: Sales Order {0} already exists against Customer's Purchase Order {1}").format(
-						so[0][0], self.po_no
+				):
+					frappe.msgprint(
+						_("Warning: Sales Order {0} already exists against Customer's Purchase Order {1}").format(
+							frappe.bold(so[0][0]), frappe.bold(self.po_no)
+						)
+					)
+				else:
+					frappe.throw(
+						_(
+							"Sales Order {0} already exists against Customer's Purchase Order {1}. To allow multiple Sales Orders, Enable {2} in {3}"
+						).format(
+							frappe.bold(so[0][0]),
+							frappe.bold(self.po_no),
+							frappe.bold(_("'Allow Multiple Sales Orders Against a Customer's Purchase Order'")),
+							get_link_to_form("Selling Settings", "Selling Settings"),
+						)
 					)
-				)
 
 	def validate_for_items(self):
 		for d in self.get("items"):
diff --git a/erpnext/selling/doctype/sales_order/test_sales_order.py b/erpnext/selling/doctype/sales_order/test_sales_order.py
index ced1ac62729..608e23a8268 100644
--- a/erpnext/selling/doctype/sales_order/test_sales_order.py
+++ b/erpnext/selling/doctype/sales_order/test_sales_order.py
@@ -2023,7 +2023,7 @@ def make_sales_order(**args):
 	so.company = args.company or "_Test Company"
 	so.customer = args.customer or "_Test Customer"
 	so.currency = args.currency or "INR"
-	so.po_no = args.po_no or "12345"
+	so.po_no = args.po_no or ""
 	if args.selling_price_list:
 		so.selling_price_list = args.selling_price_list
 
diff --git a/erpnext/stock/doctype/delivery_note/test_delivery_note.py b/erpnext/stock/doctype/delivery_note/test_delivery_note.py
index 5d8efd5d9dc..2565d1b76d1 100644
--- a/erpnext/stock/doctype/delivery_note/test_delivery_note.py
+++ b/erpnext/stock/doctype/delivery_note/test_delivery_note.py
@@ -728,7 +728,7 @@ class TestDeliveryNote(FrappeTestCase):
 
 	def test_dn_billing_status_case1(self):
 		# SO -> DN -> SI
-		so = make_sales_order()
+		so = make_sales_order(po_no="12345")
 		dn = create_dn_against_so(so.name, delivered_qty=2)
 
 		self.assertEqual(dn.status, "To Bill")
@@ -755,7 +755,7 @@ class TestDeliveryNote(FrappeTestCase):
 			make_sales_invoice,
 		)
 
-		so = make_sales_order()
+		so = make_sales_order(po_no="12345")
 
 		si = make_sales_invoice(so.name)
 		si.get("items")[0].qty = 5
@@ -799,7 +799,7 @@ class TestDeliveryNote(FrappeTestCase):
 
 		frappe.db.set_value("Stock Settings", None, "allow_negative_stock", 1)
 
-		so = make_sales_order()
+		so = make_sales_order(po_no="12345")
 
 		dn1 = make_delivery_note(so.name)
 		dn1.get("items")[0].qty = 2
@@ -845,7 +845,7 @@ class TestDeliveryNote(FrappeTestCase):
 		from erpnext.accounts.doctype.sales_invoice.sales_invoice import make_delivery_note
 		from erpnext.selling.doctype.sales_order.sales_order import make_sales_invoice
 
-		so = make_sales_order()
+		so = make_sales_order(po_no="12345")
 
 		si = make_sales_invoice(so.name)
 		si.submit()
@@ -1211,6 +1211,10 @@ class TestDeliveryNote(FrappeTestCase):
 
 		self.assertTrue(return_dn.docstatus == 1)
 
+	def tearDown(self):
+		frappe.db.rollback()
+		frappe.db.set_single_value("Selling Settings", "dont_reserve_sales_order_qty_on_sales_return", 0)
+
 
 def create_delivery_note(**args):
 	dn = frappe.new_doc("Delivery Note")