How do I get nthe EstimatedWaitTime in CallBackAPI in TaskRouting (PCCE 11.5 SandBox)?

Whenever a task is created, it returns a refURL in header for polling. Now according to the documentation, if task status is "Queued" it should return estimatedWaitTime in response but it doesn't return. Is there any configuration I need to do or need to update the url etc?

That is correct. In the response to GET (polling) API request, there will be a tag called <estimatedWaitTime/> that will indicate the estimated wait time (in seconds) as reported by the CCE router, as long as the task is in QUEUED state.