There can sometimes be some strange behaviors when changing variables/nodes; the reason you’re getting errors in your original post is due to “Get All Actors of Class” returning an array, while your “Turn On Lamp” function expects a target of just one of its members.
As ClockworkOcean said, you could just create copies of the Switch and Lamp, but you’ll fill up your folder real quick with redundant blueprints.
If you want to either control Specific Lamps with Specific Switches, i.e. communicate between actors, you need to find a way to identify which “FootLampLight” Actors Correspond to which switch, in this case the button in your Widget.
Try something like this:
For more you can check out -this post- ; Something like this is how you would set up a door button/light switch and identify which door/light the button corresponds too. Doing it this way improves scalability as you add more doors/lights/buttons to your levels. Just replace the overlap events with your button click.