how do i handle such an error?

UnityException: Tag: Ball is not defined!
Enemy.FixedUpdate () (at Assets/Enemy.cs:22) how do i handle such error in a pong game

Have you checked :

  1. That you have created and correctly named the tag “Ball”.

  2. That the Ball object has this Tag “Ball” selected from the list.

Remember Tags are not the same thing as Names?