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

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](https://github.com/dotnet/wcf/files/14672620/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?