When you go to create a new world, you can adjust your world/level type using the "World Type" dropdown menu that appears after you've entered in your world name.

If you've uploaded a pre-existing world that requires a certain level type, you can also adjust this setting by going to "Customize" > "Settings" and adjusting the "World Type" dropdown selection there. Once done, be sure to save your selection using the button at the bottom of the settings list then restart your server after you've uploaded the applicable world.

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