@onewingedcaius: tutkarz is correct. You have set up the reference to reference your character actors, but you have not specified what specific actor to get the variables from. As it stands Unreal has no idea how to choose from all the potential characters placed in your game. There are several ways to get a reference, and the best one depends on what you’re doing. If there will never be more than one of a certain actor in the world I often use Get All Actors of Class and get index 0.
For getting references to units in ATBTT, the pawn array is useful. You can use traces and collision to check find actors in certain locations or loop through all actors of a class and search for one that meets a certain criteria. In any case you will need to tell UE4 to reference a specific actor. In your second screenshot you can see that Character Reference in Default Value is set to none, so you will need to change that either directly or through code. Event Begin Play, as suggested by tutkarz is what I often use myself.