this is way more complex than one expects. I think you need to encapsulate functionality for each board square. for example a board square may know if a piece is on top, what that piece, if it allows other pieces to jump over, or if you can capture it, stuff like that.
or you may use a different approach, create a top down manager that keeps track of the board and the pieces.
I suggest finding tutorials about how to create a chess game not necessarily for unreal, and figure out the logic first then get back and try to adapt on unreal.
you can also look into turn based games logic, they have a board and various patterns on how to deal with units and the like. not necessarily for a chess game but same stuff should apply