You need to define the FollowPlayer method in your GameManager creative class. So the same way you have declared InitPlayers and InitEnemies, add the following:
FollowPlayer(Enemy:creative_prop, Player:agent):void=
// code to follow the player
This obviously doesn’t do anything but it should remove the error as the method is now defined. It’s under the assumption that Enemy is a creative_prop based on the @editable property declaration and Player is an agent type.
I’m guessing the tutorial must cover the FollowPlayer method logic, I haven’t watched it myself so I don’t know the format of the tutorial, but it may be a good idea to run through it again so see if there’s anything you might have missed.