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I'm trying to install a mod through forge however when installing it, it always crashes with a message saying "It looks like you are using an unsupported modified version of the game."

Heres the crash report:

---- Minecraft Crash Report -------- Minecraft Crash Report ----// On the bright side, I bought you a teddy bear!
Time: 6/15/17 9:40 PMDescription: There was a severe problem during mod loading that has caused the game to fail
net.minecraftforge.fml.common.LoaderExceptionModCrash: Caught exception from Pixelmon (pixelmon)Caused by: java.lang.NoClassDefFoundError: net/minecraft/stats/Achievement at com.pixelmonmod.pixelmon.config.PixelmonBlocks.load(PixelmonBlocks.java:698) at com.pixelmonmod.pixelmon.config.PixelmonConfig.loadConfig(PixelmonConfig.java:455) at com.pixelmonmod.pixelmon.config.PixelmonConfig.init(PixelmonConfig.java:448) at com.pixelmonmod.pixelmon.Pixelmon.preInit(Pixelmon.java:102) 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:483) at net.minecraftforge.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:641) 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:483) at com.google.common.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:91) at com.google.common.eventbus.Subscriber$SynchronizedSubscriber.invokeSubscriberMethod(Subscriber.java:150) at com.google.common.eventbus.Subscriber$1.run(Subscriber.java:76) at com.google.common.util.concurrent.MoreExecutors$DirectExecutor.execute(MoreExecutors.java:399) at com.google.common.eventbus.Subscriber.dispatchEvent(Subscriber.java:71) at com.google.common.eventbus.Dispatcher$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:116) at com.google.common.eventbus.EventBus.post(EventBus.java:217) at net.minecraftforge.fml.common.LoadController.sendEventToModContainer(LoadController.java:252) at net.minecraftforge.fml.common.LoadController.propogateStateMessage(LoadController.java:230) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:483) at com.google.common.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:91) at com.google.common.eventbus.Subscriber$SynchronizedSubscriber.invokeSubscriberMethod(Subscriber.java:150) at com.google.common.eventbus.Subscriber$1.run(Subscriber.java:76) at com.google.common.util.concurrent.MoreExecutors$DirectExecutor.execute(MoreExecutors.java:399) at com.google.common.eventbus.Subscriber.dispatchEvent(Subscriber.java:71) at com.google.common.eventbus.Dispatcher$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:116) at com.google.common.eventbus.EventBus.post(EventBus.java:217) at net.minecraftforge.fml.common.LoadController.distributeStateMessage(LoadController.java:147) at net.minecraftforge.fml.common.Loader.preinitializeMods(Loader.java:648) at net.minecraftforge.fml.client.FMLClientHandler.beginMinecraftLoading(FMLClientHandler.java:276) 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:483) at net.minecraft.launchwrapper.Launch.launch(Launch.java:135) at net.minecraft.launchwrapper.Launch.main(Launch.java:28)Caused by: java.lang.ClassNotFoundException: net.minecraft.stats.Achievement 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) ... 45 moreCaused by: java.lang.NullPointerException at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:182) ... 47 more

A detailed walkthrough of the error, its code path and all known details is as follows:---------------------------------------------------------------------------------------
-- System Details --Details: Minecraft Version: 1.12 Operating System: Windows 7 (amd64) version 6.1 Java Version: 1.8.0_25, Oracle Corporation Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation Memory: 208556576 bytes (198 MB) / 566276096 bytes (540 MB) up to 1060372480 bytes (1011 MB) JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0 FML: MCP 9.40 Powered by Forge 14.21.0.2330 5 mods loaded, 5 mods active States: 'U' = Unloaded 'L' = Loaded 'C' = Constructed 'H' = Pre-initialized 'I' = Initialized 'J' = Post-initialized 'A' = Available 'D' = Disabled 'E' = Errored UCH minecraft{1.12} [Minecraft] (minecraft.jar) UCH mcp{9.19} [Minecraft Coder Pack] (minecraft.jar) UCH FML{8.0.99.

Pixelmon is not out for 1.12

I have the same exact problem, I'm tempted to just re download the old minecraft launcher but I would like someone to verify if that would work...