How do I create a flat world?
Getting started, you will want to turn off your server. Once your server is off, you can proceed with setting your server on a flat world.
Set a new world name on the main page of your server control panel.

Once you set the world name, you will want to hit the save button at the bottom.
Select the files drop down button, then select the config files option.

Once on the config files page, you will want to click into the Server Settings..
Find the level-type option. Then input FLAT into it.

Scroll to the bottom and select save. You can then boot the server for the new flat world to load.
Set a new world name on the main page of your server control panel.

Once you set the world name, you will want to hit the save button at the bottom.
Select the files drop down button, then select the config files option.

Once on the config files page, you will want to click into the Server Settings..
Find the level-type option. Then input FLAT into it.

Scroll to the bottom and select save. You can then boot the server for the new flat world to load.
Updated on: 09/15/2019
Thank you!