How to check if actor cannot move to Target point?

I have dynamically spawned actors (in RTS mode) that move to 5 different waypoints on the map. They also generate the navmesh thing dynamically because you’re going to be building stuff around these actors. However, I would like to know how check if those actors cannot reach a target point, so that I could deny the player’s ability to build on that certain spot.

So basically, if you want to build something that blocks the path of the actors to the target point, then give a prompt error like “You cannot block the path.” eventually, not letting the user/player build there.

Thanks.