What this error saying and how to resolve this?

The operation ProcessPayment either has a parameter or a return type that is attributed with MessageContractAttribute.  In order to represent the request message using a Message Contract, the operation must have a single parameter attributed with MessageContractAttribute.  In order to represent the response message using a Message Contract, the operation's return value must be a type that is attributed with MessageContractAttribute and the operation may not have any out or ref parameters.

<?xml version="1.0"?>
<configuration>

  <system.web>
    <compilation debug="true" targetFramework="4.0" />
  </system.web>
  <system.serviceModel>

    <bindings>
      <basicHttpBinding>
        <binding name="SOAPBinding">
          <security mode="None"/>
        </binding>

      </basicHttpBinding>
      <webHttpBinding>
        <binding name="XMLBinding"/>
        <binding name="JSONBinding"/>
      </webHttpBinding>
    </bindings>

    <services>

      <service name="BillingApi.ProductPaymentProcessing" behaviorConfiguration="ServiceBehaviorConfig">
        <endpoint address="" binding="basicHttpBinding" contract="BillingApi.IProductPaymentProcessing" bindingConfiguration="SOAPBinding"  />
        <endpoint address="XML" binding="webHttpBinding" contract="BillingApi.IProductPaymentProcessing" behaviorConfiguration="XMLEndpointConfig" bindingConfiguration="XMLBinding"  />
        <endpoint address="JSON" binding="webHttpBinding" contract="BillingApi.IProductPaymentProcessing" behaviorConfiguration="JSONEndpointConfig" bindingConfiguration="JSONBinding"  />
        <endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange"/>

      </service>
    </services>

    <behaviors>

      <endpointBehaviors>
        <behavior name="XMLEndpointConfig">
          <webHttp automaticFormatSelectionEnabled="false" helpEnabled="true"  defaultOutgoingResponseFormat="Xml"/>
        </behavior>

        <behavior name="JSONEndpointConfig">
          <!--<enableWebScript/>-->
          <webHttp automaticFormatSelectionEnabled="false" helpEnabled="true" defaultOutgoingResponseFormat="Json"/>
        </behavior>
      </endpointBehaviors>
      <serviceBehaviors>

        <behavior name="ServiceBehaviorConfig">
          <serviceMetadata httpGetEnabled="true"/>
          <serviceDebug includeExceptionDetailInFaults="true"/>
        </behavior>

      </serviceBehaviors>
    </behaviors>
    <serviceHostingEnvironment multipleSiteBindingsEnabled="true" />

    <!--<standardEndpoints>


    </standardEndpoints>-->

  </system.serviceModel>
  <system.webServer>
    <modules runAllManagedModulesForAllRequests="true"/>
  </system.webServer>

</configuration>