Hey, sorry for the terribly late answer. I’ve been very busy these last days, but things are getting a bit better now. Glad my previous solution worked well! So what you are asking now is unfortunately not trivial, but it can absolutely be done. I have made such a feature on a couple of occasions before, but I haven’t found one that is general enough that I have included it in the toolkit.
So there are a few different ways to go about doing this. One is to have all of the functionality contained within the ability your units are using. You can make it so that when you click an enemy (which you can check in ServerInteract of the ability) you would call on the GridUI of that ability to display the attack range of the enemy, similar to how it is done on Activate for displaying the attack range of the player.
This is not terribly difficult to set up, but a bit awkward if you have many different abilities and need this behavior to be accessible to all of them. You can of course add this functionality directly to the parent ability (BP_Ability) to make it accessible.
There are other ways to achieve an effect like this as well, but what is best depends on your game. Let me know if you have any further questions on this.
You use the same animations for all, but I assume they are each using their own animation blueprint?
For your error it is probably because you ahve filled the Abilitites array with several empty values. Make sure every item in the array is a valid ability class.