You’re on the right track, Hinato 
The only problem I can see here is that you’ve incorrectly referenced the Grid Manager. Instead of casting to BP_Grid_Manager cast to ATBTT_PlayerController and from the output node of the cast get a reference to the GridManagerRef variable. From this variable get the edge array and proceed like you’ve done. Remenber to be finished with all alterations to the edge array before you run the pathfinding.