How action points are handled is pretty much up to the ability to define. If you never want to have a move longer than one tile, then set the move input value of the RunPathfinding function that runs on ability activation to 1. That should solve the move range problem.
Now if I understand you correctly you want the player to be able to do two things: First move a maximum of one tile and then build a block. You could set this up using action points in a similar way to how I’ve done it in BP_Ability_MoveAttack. Set units up to have 2 AP by default. On ability activation add a switch on the current AP value of the owning unit. If it is 2, run pathfinding with a range of one, display the tiles etc. If it is 1, instead show the tiles you are allowed to build on. You could potentially do this in a similar way to how BP_Ability_Attack activates.
Then in the click event graph again check the current AP of the unit. If it is one, proceed with checking if the clicked tile can be moved to and move to it if it can. If AP is 1 instead check whether it is allowed to build a block on the selected tile and proceed with whatever code you have for doing this if true.