Just let it generate a default world.I have been trying to get Biomes O' Plenty to generate a world on my server but if I put BIOMESOP in the level type it still generates a vanilla world. I have also tried creating a singleplayer world and transferring it over and it results in either missing chunks or just crashes. I would love if anyone had any idea on how to install BoP properly.
But first, go into the Biomes o Plenty config and set the "generate in default worlds" to true. You can do the same for Highlands config if you want those biomes.