I opened Pixelmon on 5.1.1, (also tried this on 5.0.2), and got to the 7/7 when loading Forge. However, when it got to "Reloading TextureManager" Part 3, my game suddenly crashed. I cannot get it to work, and the crash log will be below.

---- Minecraft Crash Report ----
// Would you like a cupcake?

Time: 6/20/17 12:10 PM
Description: 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.NoSuchMethodError: com.pixelmonmod.pixelmon.items.PixelmonItemBlock.func_77655_b(Ljava/lang/String;)Lnet/minecraft/item/ItemBlock;
at com.pixelmonmod.pixelmon.items.PixelmonItemBlock.<init>(PixelmonItemBlock.java:54)
at com.pixelmonmod.pixelmon.config.PixelmonItems.load(PixelmonItems.java:449)
at com.pixelmonmod.pixelmon.config.PixelmonConfig.loadConfig(PixelmonConfig.java:456)
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.EventSubscriber.handleEvent(EventSubscriber.java:74)
at com.google.common.eventbus.SynchronizedEventSubscriber.handleEvent(SynchronizedEventSubscriber.java:47)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:322)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:304)
at com.google.common.eventbus.EventBus.post(EventBus.java:275)
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.EventSubscriber.handleEvent(EventSubscriber.java:74)
at com.google.common.eventbus.SynchronizedEventSubscriber.handleEvent(SynchronizedEventSubscriber.java:47)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:322)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:304)
at com.google.common.eventbus.EventBus.post(EventBus.java:275)
at net.minecraftforge.fml.common.LoadController.distributeStateMessage(LoadController.java:147)
at net.minecraftforge.fml.common.Loader.preinitializeMods(Loader.java:647)
at net.minecraftforge.fml.client.FMLClientHandler.beginMinecraftLoading(FMLClientHandler.java:276)
at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:440)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:352)
at net.minecraft.client.main.Main.main(SourceFile:124)
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)


A detailed walkthrough of the error, its code path and all known details is as follows:
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-- System Details --
Details:
Minecraft Version: 1.11.2
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: 260278008 bytes (248 MB) / 767098880 bytes (731 MB) up to 4281597952 bytes (4083 MB)
JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx4G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
FML: MCP 9.38 Powered by Forge 13.20.0.2315 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{1.11.2} [Minecraft] (minecraft.jar)
UCH mcp{9.19} [Minecraft Coder Pack] (minecraft.jar)
UCH FML{8.0.99.99} [Forge Mod Loader] (forge-1.11.2-13.20.0.2315.jar)
UCH forge{13.20.0.2315} [Minecraft Forge] (forge-1.11.2-13.20.0.2315.jar)
UCE pixelmon{5.1.1} [Pixelmon] (Pixelmon-1.10.2-5.1.1-universal.jar)
Loaded coremods (and transformers):
GL info: ' Vendor: 'Intel' Version: '2.1.0 - Build 8.15.10.2202' Renderer: 'Intel(R) HD Graphics'