Check out Epic’s Blueprint Communications level in ContentExamples, they showed how to do a button which tells a light to go on and off. For your “only once” setup, you can just have a boolean variable inside the Switch blueprint. Once you Interact with the switch, you check if that variable is true, if not set it to true and do your light functions. Then, on the next interaction, it will see that the variable is already true, and do nothing. As I said, check out the Blueprint Communications in ContentExamples, they will help you! Good Luck!
Ive got two questions one how would you be able to make. A button the player can only use once and two how would you attach that to light to light up the building