dotnet-svcutil Error: Nested groups may not be used with encoded SOAP.

[ramiabughazaleh]

When trying to generate client code for a WCF service, the following error occurs:
Error: Nested groups may not be used with encoded SOAP. Please change definition of schema type 'PromotionCouponSingleOrMultiple' from namespace 'urn:order'.

Steps to reproduce

1. Download and extract AvangateServiceReference.zip
2. Run dotnet-svcutil AvangateServiceReference.wsdl

For reference, the original wsdl can be found here: https://api.avangate.com/soap/6.0/?wsdl

Expected behavior
I expected a c# proxy client to be generated.
Otherwise, the error message doesn't indicate what steps are needed so that a c# proxy client can be generated.
Is there an example?