Hi @Rajeev_Lochan_Subs - the webhook response when calling an inbound webhook connector is expected to be empty by default. You can configure a response body expression to return data if you need.
The second question is not related to Camunda. That is a SendGrid error, and it has been many, many years since I personally used SendGrid, so I am not sure. I would check their documentation about dynamic vs legacy template IDs.