How to use a structure block & structure void?
Solution 1:
Structure blocks are how you can save a certain region of blocks to a file or load from files into your world. A GUI is a graphical user interface, sort of like the thing that opens when you open a chest.
When the wiki says:
When placed, use the structure block to open the structure block GUI. The GUI opens only if the player is in Creative mode, and has the proper permissions.
It means:
You can right-click on the structure block to open its interface. You can only do so if you are in Creative Mode and have the proper permissions.