Hi Shankar,
Some public test servers expose the functionality that you are looking for, where for example a Patient request without any search parameters would return a list of every known patient. Cerner's public sandbox does not expose this, for a few reasons: there are lots of patients in the system and such requests would be very resource-hungry, and this behavior would not be available in production deployments so we have opted not to support it in our sandbox either.
If you want a list of every patient in the sandbox, that is not available via FHIR APIs.