Hello, I'm having an issue with a fresh install of OTG, i just created a server, followed the instructions, and every time it gives me this error when i reset the world when trying to fix the issue:
(I've tried redownloading, re-extracting, and renaming the world folder and in the configs like the instruction shows.)

[17:07:32] [Server thread/ERROR]: [OpenTerrainGenerator] Could not set generator for default world 'Biome Bundle': Plugin 'OpenTerrainGenerator v1.12.2 v6
java.lang.ExceptionInInitializerError: null
at com.pg85.otg.bukkit.OTGBiomeBase.createInstance(OTGBiomeBase.java:131) ~[?:?]
at com.pg85.otg.bukkit.BukkitBiome.forCustomBiome(BukkitBiome.java:43) ~[?:?]
at com.pg85.otg.bukkit.BukkitWorld.createBiomeFor(BukkitWorld.java:171) ~[?:?]
at com.pg85.otg.bukkit.BukkitWorld.createBiomeFor(BukkitWorld.java:1646) ~[?:?]
at com.pg85.otg.configuration.ServerConfigProvider.indexSettings(ServerConfigProvider.java:264) ~[?:?]
at com.pg85.otg.configuration.ServerConfigProvider.loadBiomes(ServerConfigProvider.java:135) ~[?:?]
at com.pg85.otg.configuration.ServerConfigProvider.loadSettings(ServerConfigProvider.java:79) ~[?:?]
at com.pg85.otg.configuration.ServerConfigProvider.(ServerConfigProvider.java:68) ~[?:?]
at com.pg85.otg.bukkit.OTGPlugin.getDefaultWorldGenerator(OTGPlugin.java:137) ~[?:?]
at org.bukkit.craftbukkit.v1_12_R1.CraftServer.getGenerator(CraftServer.java:1224) [spigot-1.12.2.jar:git-Spigot-b6ecf3b-68b4fb6]
at net.minecraft.server.v1_12_R1.MinecraftServer.a(MinecraftServer.java:260) [spigot-1.12.2.jar:git-Spigot-b6ecf3b-68b4fb6]
at net.minecraft.server.v1_12_R1.DedicatedServer.init(DedicatedServer.java:272) [spigot-1.12.2.jar:git-Spigot-b6ecf3b-68b4fb6]
at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:545) [spigot-1.12.2.jar:git-Spigot-b6ecf3b-68b4fb6]
at java.lang.Thread.run(Unknown Source) [?:?]
Caused by: java.lang.RuntimeException: java.lang.NoSuchMethodException: sun.reflect.ReflectionFactory.newConstructorAccessor(java.lang.reflect.Constructor)
at com.pg85.otg.bukkit.util.EnumHelper.(EnumHelper.java:39) ~[?:?]
... 14 more
Caused by: java.lang.NoSuchMethodException: sun.reflect.ReflectionFactory.newConstructorAccessor(java.lang.reflect.Constructor)
at java.lang.Class.getDeclaredMethod(Unknown Source) ~[?:?]
at com.pg85.otg.bukkit.util.EnumHelper.(EnumHelper.java:33) ~[?:?]
... 14 more

For anyone who still has this issue, I fixed it by switching from Java 17 to Java 8 on my 1.12.2 server.

Looks like this is caused by incorrect installation, probably by not setting world type to OTG.

No issues like this have been reported by other users so closing this for now.

I have the same issuse

I have confirmed the issue is a problem with the java instillation, OpenJDK8 is not compatible, make sure your java version is the latest

except i don't use OpenJDK8.. i use JRE8, and i'm positive it was up to date at the time of posting.