The invalid operation exception you face is likely because you need to fill in the Data property with Base64 encoded bytes like  sv.GetOrCreate(x => x.Activities.Activity3_operation.Request.BinaryContent).Data = System.Convert.ToBase64String(myPreparedBytes);

I cannot see anything overriding . Do we have any Global Content-Type in SV tool?

Anyone here to help me?

From the Simulation Report it is apparent that the Content Type is set from Rule number 4 to text/plain. But you say that in fact there is some other value being produced on output? Could you please attach a screenshot from the Message Log with that message recorded in simulation? 

Authorization: Bearer qwertyuiopasdfhdkndlfkvnkgbkrdgjnlkdjfnvlkdsmfnldmfnlsdkmnflksmrnflskejfls,kjfls,dfglksdjfghlskdjfghlskjdhflksjhdfgkefjhslfdgkjshlfdkjhghslkjfghlskdjfghlksjdfhlgksjdhflkgjshldkfjghlskdjfh
Content-Type: application/json; charset=utf-8

AgentType: HTTP
MessageId: 00000000-0000-0000-0000-000000000000
MessageType: Request
Method: POST
OperationDiscriminators: UriPath = **
ServiceId: 
ServiceMode: Simulating

{#JSON#}

But this is request, not response. We simulate responses, also the simulation report shows how is a response constructed. Request we receive on input, so the client who sent it was responsible for setting the content type. Or is this a SCA request?

Sorry I need a screenshot, not just text.

This is SCA Call (notification) that we need to send after 30 secs. For that we need to send it as text/plain or application/vnd.kafka.json.V2+json. But from SCA call option, We are not able to do it. It is automatically changing to Content-Type: application/json; charset=utf-8.

OK, but this screenshot shows request of a different SCA than you showed in the Simulation Report. This one is called "SCA **" and the other one is called "SCA Async Success"