1. I am working on HR onboarding process where the candidate can give feedback a

Have your capture feedback quick task in the separate sub process and plug this new sub process in place of where your feedback quick task is currently present. So, now when your this sub process triggers, use modify process instance security smart service (essentially before the feedback quick task is assigned) in your new subprocess and modify your instance security to the appropriate user based on assignment.

In this way only the appropriate user will be seeing their respective quick tasks in the related actions.
PS: The security of the related action in process backed is governed by the process security.
Hope it solves your problem.

Appreciated if you can help me in configuration of "Modify Process Security" smart service. it shows admin views etc option where i need to pass user type pv

Yes, the modify process instance security asks for various rights that you can configure according to the required use cases. In your case if the user to whom task is assigned can work just provide that user in the 'Viewer' role. For more details, have a look at below link.
forum.appian.com/.../Modify_Process_Security_Smart_Service.html

I added the list of users to Explicit Non Viewers. but still related action is visible to these users..

Try adding only the relevant user based on assignment in Viewers input only, And once you run the instance and to ensure that the correct security is configured for the process instance, under the process tab of the designer select that instance and click on Security to check that the security is as per what you have configured in the service.

I am getting below error as my end users are basic users to work on flow..

There is a problem with task “Modify Process Security” in the process “Add Workflow”. User does not have permissions to update security.
Any inputs..

Please make sure that under the Assignment tab of your Modify process Security smart service, you configure it as "Whosoever designed the process model" as basic user would not have the sufficient privileges to modify the process security.