In defaults you can only select actors that are inside level placed in editor, not once that are spawn during gameplay, because blueprint system cant name them until they are spawned. So you could place task like a object on the level, would be good if you would add atleast billboard component to your task class so you can see it on level in editor. If you use ATaks as base class for diffrent tasks that you spawn, you could use UClass type or more specificly:
SubclassOf<ATask>
which will limit selection in editor to specific base class and then use it to spawn content of that array
Other then that you would need to use iterators to get unrealated actor that been spawn during gameplay, you need to do that when the object will be spawned
Blueprint has equivlent of that too