Hi @PRAKHAR BHARDWAJ ,
At this time, if your cluster has already been deployed, you cannot use Windows Admin Center to re-deploy just the SDN environment. Doing so from Windows Admin Center can currently only be done during the initial deployment.
To add SDN to your environment, the best way to do so is to follow this guide - https://learn.microsoft.com/en-us/azure-stack/hci/manage/sdn-express. This guide uses the SDN Express scripts to do the deployment.
More information on SDN on HCI can be found here - https://learn.microsoft.com/en-us/azure-stack/hci/concepts/software-defined-networking.
Hope this helps!
Trent