How to make the NPC following players in multiplayer game

I following tutorial here to make the NPC or enemy character chasing the player but the NPC can't detect where the player is. The player is a prefabs that not placed in the scene, it calls when the game start. So when I'm trying another object in the scene and make it as a an object to follow by NPC, the NPC can follow it. Please help me to fix it, I'm new to game development. And because its a multiplayer game, can the NPC choose which player to chase?