Configure the server properties
Navigate to the root directory of your server (where the Spigot.jar file is located). Open the server.properties file using a text editor such as Notepad++ or Sublime Text.
Edit the world generation settings
Look for the following line in the server.properties file:
Change the value from default to flat as follows:
This change tells the server to create a flat world for the default overworld.
Delete the old world (optional)
If you have already created a world, you must delete it for the new flat world settings to take effect. Locate the folder named world in your server directory and delete it. Be careful, as this will permanently delete your previous world and all progress made in it. It is recommended that you make a backup copy before performing this step.
Start the server
Restart your Spigot server. The server will now generate a flat world as the main parent world.
Note that this process only affects the overworld. If you want to create a flat world for the Nether or End dimensions, you will need to use a plugin like "Multiverse Core" or "WorldEdit" to create and manage these dimensions separately.
How to create a flat world with the Multiverse-Core plugin?
Multiverse-Core is a popular plugin for Spigot that allows you to create and manage multiple worlds on your server. Here's how to create a flat world with Multiverse-Core.
Download the latest version of the Multiverse Core plugin from the SpigotMC resource page (https://www.spigotmc.org/resources/multiverse-core.390/). Place the downloaded .jar file in the plugins folder in the root directory of your server.
Restart your server
Restart your server so that Multiverse Core is loaded. The server will create a Multiverse Core folder in the plugins folder.
Configure the flat world
To create a new flat world, use the following command in-game or from the server console:
/mv create <world_name> normal -t flat
Replace <world_name> with the desired name for your flat world. The -t flat flag tells Multiverse core to create a flat world.
For example, if you want to create a flat world named "flatworld", the command is:
/mv create flatworld normal -t flat
Teleport to the flat world
After you create the flat world, you can teleport to it using the following command
/mv tp <player name> <world name>.
Replace <player name> with your name in the game (or the name of the player you want to teleport) and <world name> with the name of the flat world you created in step 3.
For example, to teleport to "flatworld", the command would be:
/mv tp your_name flatworld
Set the flat world as the default world (optional)
If you want the flat world to be the default world that players spawn in when they join your server, edit your server.properties file. Change the value of level-name to the name of the flat world you created:
Save the file and restart your server for the changes to take effect.
That's it! You have now created a flat world with Multiverse core on your Minecraft Spigot server. You can use other Multiverse commands to manage the worlds on your server, including setting specific spawn points, managing permissions, and more. See the Multiverse documentation for a complete list of commands and functions (https://github.com/Multiverse/Multiverse-Core/wiki).