Ora-31000 not an XDB schema document error WHY???

Question

I have registered a xmlschema as following
FIRSTLY
I create a procedure schemas as
create or replace procedure schemas is
begin
declare
doc varchar2(1000) := '<schema
targetNamespace="http://localhost/po.xsd"
xmlns:po="http://localhost/po.xsd"
xmlns="http://www.w3.org/2001/XMLSchema"
xmlns:xdb="http://xmlns.oracle.com/xdb">
<complexType name="PurchaseOrderType">
<sequence>
<element name="PONum" type="decimal"/>
<element name="Company">
<simpleType>
<restriction base="string">
<maxLength value="100"/>
</restriction>
</simpleType>
</element>
<element name="Item" maxOccurs="1000">
<complexType>
<sequence>
<element name="Part">
<simpleType>
<restriction base="string">
<maxLength value="1000"/>
</restriction>
</simpleType>
</element>
<element name="Price" type="float"/>
</sequence>
</complexType>
</element>
</sequence>
</complexType>
<element name="PurchaseOrder" type="po:PurchaseOrderType"/>
</schema>';
begin
dbms_xmlschema.registerSchema('http://localhost/po.xsd', doc);
end;
end schemas;
SECONDLY
I execute the schemas procedure as
"exec schemas;"
and the return value show is successful
THIRDLY
I want to create a table based on this registered schema as
"create table po_tab(
id number,
po sys.XMLType
xmltype column po
XMLSCHEMA "http://localhost/PO.xsd"
element "PurchaseOrder";
But the return value is
"ORA-31000: the 'http://localhost/PO.xsd' is not an XDB schema document "
WHY?????

Answer 1

XML is case sensitive.. You used ''http://localhost/po.xsd' when registering the schema and "http://localhost/PO.xsd" when attempting to create the table. Which database version are you working with