Why my phone is not vibrating?

@somanshsaxena since you are not giving us a lot of background information, we'll have to start with the easy things. Could be a matter of not having it enabled. Go to Settings > Sounds and vibration > System sound/Vibration control. Enable the required toggle switches and see if your vibrate option now works.

Of course, you can also try to get into the diagnostics mode by dialing *#0*# and run the Vibration test and see if it passes. If it does not, you will need to replace the vibrator motor.