My minecraft server world won't set to SUPERFLAT like I want it to?

So I made a minecraft server for me and my friends, but for some reason, I can't get it to be set as superflat. I'm wondering If I'm doing something wrong. Can I please have some help?!

You have to specify this in the server.properties file.

Open your server.properties file in Notepad and find "level-type", then replace 'DEFAULT' with 'FLAT'. Delete the generated world if you haven't already and launch the server.

If done correctly, the world will generate a superflat file. The alternative would be to create a superflat world in single player, then transferring that file to your server.