I have the following service definition (incomplete, but demonstrates the error. Visual Studio tells me (translated from german) "In this Schema Namespace http://schemas.xmlsoap.org/wsdl/ is not available" at the line where I define OffertXML.
<?xml version="1.0" encoding="UTF-8"?>
<wsdl:definitions name="TestOffertverwaltung"
targetNamespace="urn:TestOffertverwaltung"
xmlns:tns="urn:TestOffertverwaltung"
xmlns:xsd="http://www.w3.org/2001/XMLSchema"
xmlns:types="urn:Test/types"
xmlns:SOAP="http://schemas.xmlsoap.org/wsdl/soap/"
xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/"
xmlns:xmime="http://www.w3.org/2005/05/xmlmime"
xmlns="http://schemas.xmlsoap.org/wsdl/">
<wsdl:types>
<xsd:schema targetNamespace="urn:Test/types">
<xsd:element name="OffertXML" type="string"/>
<xsd:element name="OffertPDF" xmime:expectedContentTypes='image/jpeg' type="xmime:base64Binary"/>
<xsd:element name="OffertMeta" type="string"/>
<xsd:element name="OffertId" type="string"/>
</xsd:schema>
</wsdl:types>
<!-- ... -->
</wsdl:definitions>