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

Unless I’m mistaken that would be the expected behavior at this point. If you added a troubleshooting keyboard event that would switch the State between chase player and attack player you would be roughly at the end of the second part of that tutorial series. Part 3 would then add perception to the enemy so that they need to see the player to attack and would check out the last point of contact etc.