Hi Bharani,
To use the Function keys, you need to assign them with user commands first.
Do the following steps:
1. Double click on you GUI status
2. Go to screen menu: Utillities --> F Key Consistency
3. Enter the command for the Function keys you desire to use.
4. Save and Activate it
Note the F keys assignment will be visible in black colour in the GUI status screen here after.
Please try and let me know if it helps