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I've been trying to download Pixelmon for an hour now and cannot figure out why whenever I run Forge on Minecraft why I get this error.

The game crashed whilst initializing game
The game crashed whilst initializing game
Error: java.lang.NoClassDefFoundError: ckc

Crash Report:

---- Minecraft Crash Report -------- Minecraft Crash Report ----
WARNING: coremods are present:Contact their authors BEFORE contacting forge
// Why is it breaking
Time: 9/8/18 7:47 PMDescription: Initializing game
java.lang.NoClassDefFoundError: ckc at java.lang.Class.getDeclaredFields0(Native Method) at java.lang.Class.privateGetDeclaredFields(Class.java:2583) at java.lang.Class.getField0(Class.java:2975) at java.lang.Class.getField(Class.java:1701) at net.minecraftforge.fml.client.FMLClientHandler.detectOptifine(FMLClientHandler.java:286) at net.minecraftforge.fml.client.FMLClientHandler.beginMinecraftLoading(FMLClientHandler.java:209) at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:466) at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:377) at net.minecraft.client.main.Main.main(SourceFile:123) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:497) at net.minecraft.launchwrapper.Launch.launch(Launch.java:135) at net.minecraft.launchwrapper.Launch.main(Launch.java:28)Caused by: java.lang.ClassNotFoundException: ckc at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:191) at java.lang.ClassLoader.loadClass(ClassLoader.java:424) at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ... 15 moreCaused by: java.lang.NullPointerException at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:182) ... 17 more

A detailed walkthrough of the error, its code path and all known details is as follows:---------------------------------------------------------------------------------------
-- Head --Thread: Client threadStacktrace: at java.lang.Class.getDeclaredFields0(Native Method) at java.lang.Class.privateGetDeclaredFields(Class.java:2583) at java.lang.Class.getField0(Class.java:2975) at java.lang.Class.getField(Class.java:1701) at net.minecraftforge.fml.client.FMLClientHandler.detectOptifine(FMLClientHandler.java:286) at net.minecraftforge.fml.client.FMLClientHandler.beginMinecraftLoading(FMLClientHandler.java:209) at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:466)
-- Initialization --Details:Stacktrace: at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:377) at net.minecraft.client.main.Main.main(SourceFile:123) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:497) at net.minecraft.launchwrapper.Launch.launch(Launch.java:135) at net.minecraft.launchwrapper.Launch.main(Launch.java:28)
-- System Details --Details: Minecraft Version: 1.12.2 Operating System: Windows 10 (amd64) version 10.0 Java Version: 1.8.0_51, Oracle Corporation Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation Memory: 109860360 bytes (104 MB) / 268435456 bytes (256 MB) up to 2147483648 bytes (2048 MB) JVM Flags: 8 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx2G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=16M IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0 FML: Loaded coremods (and transformers): Launched Version: 1.12.2-forge1.12.2-14.23.4.2705 LWJGL: 2.9.4 OpenGL: Intel(R) HD Graphics 630 GL version 4.5.0 - Build 22.20.16.4836, Intel GL Caps: Using GL 1.3 multitexturing.Using GL 1.3 texture combiners.Using framebuffer objects because OpenGL 3.0 is supported and separate blending is supported.Shaders are available because OpenGL 2.1 is supported.VBOs are available because OpenGL 1.5 is supported.
Using VBOs: Yes Is Modded: Definitely; Client brand changed to 'fml,forge' Type: Client (map_client.txt) Resource Packs: Current Language: ~~ERROR~~ NullPointerException: null Profiler Position: N/A (disabled) CPU: 8x Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz

It says that I am using an unsupported modified version of the game.

Now I'm getting this error:

For some reason after I delete Optifine I was able to run forge.