Hi Ravi,
You would need to install "Simulink Test" + "Embedded Coder" products in order to create a "SIL test harness". Upon installing both of these, you should be able to right click on the Subsystem and see the "Test Harness" menu and should be able to select the "Verification" option to "SIL" in the Create Test Harness dialog.
Here is the link to the documentation that talks about how to go about configuring your model to create a SIL test harness:
https://www.mathworks.com/help/sltest/ug/silpil-verification-for-a-subsystem.html
