<Posting as Answer due to comment limit>
Welcome to Microsoft Q&A! Thanks for posting the question.
Can you provide more details what operation you are performing when you have observed the error. I couldn't observe the issue (version 2.1.0) when I have created the namespace, creating/updating entities. Please provide the exact request parameters value that you have passed on that operation (method called) for which you have observed the InvalidApiVersionParameter exception.
Yes you cannot set the apiversion as this is the read only property (ServiceBusManagementClient.ApiVersion) and the SDK does it for you but we need to know the operation and parameters that were passed so I repro the issue and engage my team to assist further.