I'm trying to set up a simple AI for the enemy blueprint to make them target player's chosen character which is a parent of the main character BP

[PasMonVraiNom]

Using “Get player pawn” and setting that to the Attack target blackboard key would do exactly that, even on your separate player character BP.

Try putting the delay back in and the setting of the attacktarget key. When you get to AI perception, that’s when you would take this bit out and call it from another function when your player is spotted.