If you want to turn on generating this biome without generating new world right now do this: 1. Download NBTExplorer. 2. Stop server. 3. Open level.dat file from your world folder. 4. Find Data -> WorldGenSettings -> dimensions -> minecraft:overworld -> generator -> biome_source -> preset. 5. Change the value from "minecraft:overworld" to "minecraft:overworld_update_1_20". 6. Save and start server.
MrShadow_
If you want to turn on generating this biome without generating new world right now do this:
1. Download NBTExplorer.
2. Stop server.
3. Open level.dat file from your world folder.
4. Find Data -> WorldGenSettings -> dimensions -> minecraft:overworld -> generator -> biome_source -> preset.
5. Change the value from "minecraft:overworld" to "minecraft:overworld_update_1_20".
6. Save and start server.