XML Validation error: The element is not declared

Try this:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
   <xs:complexType name="POSearch">
       <xs:sequence minOccurs="0" maxOccurs="10">
           <xs:element name="POID" type="xs:positiveInteger"/>
       </xs:sequence>
   </xs:complexType>
    <xs:element name="POSearch" type="POSearch"/>
</xs:schema>

You have the type declared but no element declared of that type.

Add an element declaration:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

   <xs:element name="POSearch" type="POSearch"/>

   <xs:complexType name="POSearch">
       <xs:sequence minOccurs="0" maxOccurs="10">
           <xs:element name="POID" type="xs:positiveInteger"/>
       </xs:sequence>
   </xs:complexType>
</xs:schema>