Hi Satish,
I'd recommend you to start with an example project based on SDK (menu File -> New -> S32DS Project from Example)

Build the imported example and start the debug session.
When you are in the debug session click on Run/Resume button.
Open FreeMASTER project (freemaster_s32k148.pmp)

Before starting FreeMASTER communication please make sure that the OpenSDA COM port is available in the system manager:

You can directly set the com port and speed in FreeMASTER menu Project -> Options so it matches with the setup running on the target board:
(FreeMASTER SDK example project has speed 115200 baud):

Now you should be able to detect the board and visualize the variables.

The message "Could not open the communication port" typically indicates the selected com port does not exist or it is been used by a different application.
Hope it helps.
Stan