I have a redstone testing singleplayer world, and when I try to re-create it, there is nothing there.
This is an issue because I want to make the world into a multiplayer server, and when I take out the file and upload it, there is nothing there. This isn't something I have done wrong as I've done it many times before.
Recreating a world makes a new world with the same seed. It doesn't copy over any changes you made to it.
Copy the world into your server folder and make sure the name of it is the same as
level-nameinserver.properties.