Change World Type

By default, your Minecraft server will have a regular world type, but you may prefer a flat world for building.

Open your browser and go to https://panel.fps.ms.
Select your server and go to Files
Search for and open the server.properties file.
Scroll down until you see the level-type option.
Replace the default type with any world type you would like (list provided below for ease).
Click on Save Content and restart your server to see the changes.

minecraft:normal - Standard world with hills, valleys, water, etc.
minecraft:flat - A flat world with no features, can be modified with generator-settings.
minecraft:large_biomes - Same as default but all biomes are larger.
minecraft:amplified - Same as default but world-generation height limit is increased.
minecraft:single_biome_surface - A buffet world which the entire overworld consists of one biome, can be modified with generator-settings.
buffet - Only for 1.15 or before. Same as default unless generator-settings is set.
default_1_1 - Only for 1.15 or before. Same as default, but counted as a different world type.
customized - Only for 1.15 or before. After 1.13, this value is no different than default, but in 1.12 and before, it could be used to create a completely custom world.