For an empty flat world, stop the server, make sure that server.properties has the level type set to FLAT (DEFAULT for survival) and that the spawn world is set to a name that doesn't already exist in the worlds folder, change pocketmine.yml default level type to pmanvil if it isn't already, and start the server.
I know, but full empty like emptyworld generator. I try to use this but plugin srash my server. I sreach how to make "one block on all map" for skybloc islands
Ok, completely empty it is! I haven't tried it in a long time so I don't guarantee it works, but it might. Add it at the end of pocketmine.yml to generate a world called 'void': Code: worlds: void: generator-settings: FLAT:2;0;1;