@ranjeet70925 start off by dialing *#0842# and select the vibrate option. If it works, do a factory reset on your phone. Make sure you back-up all of your files, contacts etc. You will lose whatever has been stored by you on the phone.
If it does not work, it is time to replace the vibration motor. Since iFixit does not yet have a guide for that, use something like [https://www.nadiemellamagallina.com/en/manuales/smartphones-1/samsung-4/samsung-galaxy-a71-1204/vibrador-10717|this guide]
While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. [link|https://ifixit.com/Guide/new|https://ifixit.com/Guide/new|new_window=true] The iFixit community will appreciate this.