During the Installation of CAS am getting the below error saying that CSS 12.3 Version or above is not found. But CSS 12.3 Version is present in the server and its got during Synthetic Enterprise manger installation.
Is the CSS Service running? did you choose the correct port to connect to the CSS? Maybe there was a different port provided within the installation of CSS.
It should be a good check if you can connect to CSS via RUM Console or via Web (http://localhost:4182/securityui )
If the CSS service is running but you cannot connect to the CSS maybe you should have a look into the CSS logs.