Fluent-style API is not supported
Starting with NServiceBus version 8, the fluent-style testing API is not supported. Tests should be written in an Arrange-Act-Assert (AAA) style. Tests written this way will create the handler or saga to be tested, and call methods on them directly, passing in a testable message handler context that will capture outgoing operations that can be asserted on afterwards.
See the upgrade guide for details on converting existing tests.