---- Minecraft Crash Report ----
// I bet Cylons wouldn't have this problem.

Time: 6/28/18 5:14 PM
Description: Rendering screen

java.lang.NoSuchMethodError: net.minecraft.client.gui.inventory.GuiInventory.func_194310_f()Lnet/minecraft/client/gui/recipebook/GuiRecipeBook;
    at com.pixelmonmod.pixelmon.client.gui.inventory.InventoryPixelmon.drawScreen(InventoryPixelmon.java:145)
    at com.pixelmonmod.pixelmon.client.gui.inventory.GuiInventoryPixelmonExtended.func_73863_a(GuiInventoryPixelmonExtended.java:101)
    at net.minecraftforge.client.ForgeHooksClient.drawScreen(ForgeHooksClient.java:349)
    at net.minecraft.client.renderer.EntityRenderer.func_181560_a(EntityRenderer.java:1115)
    at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:1117)
    at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:397)
    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)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Thread: Client thread
Stacktrace:
    at com.pixelmonmod.pixelmon.client.gui.inventory.InventoryPixelmon.drawScreen(InventoryPixelmon.java:145)
    at com.pixelmonmod.pixelmon.client.gui.inventory.GuiInventoryPixelmonExtended.func_73863_a(GuiInventoryPixelmonExtended.java:101)
    at net.minecraftforge.client.ForgeHooksClient.drawScreen(ForgeHooksClient.java:349)

-- Screen render details --
Details:
    Screen name: com.pixelmonmod.pixelmon.client.gui.inventory.GuiInventoryPixelmonExtended
    Mouse location: Scaled: (213, 119). Absolute: (427, 240)
    Screen size: Scaled: (427, 240). Absolute: (854, 480). Scale factor of 2

-- Affected level --
Details:
    Level name: MpServer
    All players: 1 total; [EntityPlayerSP['DeNatteKat'/1181, l='MpServer', x=-167.09, y=64.25, z=-242.68]]
    Chunk stats: MultiplayerChunkCache: 408, 408
    Level seed: 0
    Level generator: ID 00 - default, ver 1. Features enabled: false
    Level generator options: 
    Level spawn location: World: (-132,64,-200), Chunk: (at 12,4,8 in -9,-13; contains blocks -144,0,-208 to -129,255,-193), Region: (-1,-1; contains chunks -32,-32 to -1,-1, blocks -512,0,-512 to -1,255,-1)
    Level time: 54244 game time, 54244 day time
    Level dimension: 0
    Level storage version: 0x00000 - Unknown?
    Level weather: Rain time: 0 (now: false), thunder time: 0 (now: false)
    Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
    Forced entities: 15 total; [EntityPlayerSP['DeNatteKat'/1181, l='MpServer', x=-167.09, y=64.25, z=-242.68], EntityPixelmon['Ducklett'/1184, l='MpServer', x=-138.87, y=64.00, z=-219.31], EntityPixelmon['Pidgey'/1185, l='MpServer', x=-124.25, y=64.00, z=-231.40], EntityItem['item.tile.flower2.poppy'/2, l='MpServer', x=-114.92, y=22.00, z=-271.13], EntityPixelmon['Pidgey'/1190, l='MpServer', x=-135.59, y=66.40, z=-239.61], EntityPixelmon['Pachirisu'/1191, l='MpServer', x=-148.79, y=68.00, z=-260.50], EntityPixelmon['Magikarp'/1192, l='MpServer', x=-133.45, y=58.42, z=-185.22], EntityPixelmon['Diglett'/1195, l='MpServer', x=-172.50, y=50.00, z=-259.50], EntityPixelmon['Togetic'/1196, l='MpServer', x=-176.50, y=64.00, z=-252.50], EntityPixelmon['Frillish'/1198, l='MpServer', x=-105.35, y=49.00, z=-193.68], EntityPixelmon['Diglett'/1201, l='MpServer', x=-142.50, y=46.00, z=-286.50], EntityPixelmon['Zubat'/1202, l='MpServer', x=-132.78, y=50.74, z=-271.83], EntityPixelmon['Dedenne'/1203, l='MpServer', x=-100.50, y=65.00, z=-229.50], EntityPixelmon['Pidgey'/1204, l='MpServer', x=-117.22, y=67.67, z=-247.69], EntityPixelmon['Pidgey'/1205, l='MpServer', x=-128.50, y=64.00, z=-256.50]]
    Retry entities: 0 total; []
    Server brand: fml,forge
    Server type: Non-integrated multiplayer server
Stacktrace:
    at net.minecraft.client.multiplayer.WorldClient.func_72914_a(WorldClient.java:415)
    at net.minecraft.client.Minecraft.func_71396_d(Minecraft.java:2732)
    at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:418)
    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
    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: 331559656 bytes (316 MB) / 1073741824 bytes (1024 MB) up to 1073741824 bytes (1024 MB)
    JVM Flags: 8 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=16M
    IntCache: cache: 0, tcache: 0, allocated: 13, tallocated: 95
    FML: MCP 9.40 Powered by Forge 14.21.1.2387 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
    UCHIJA    minecraft{1.12} [Minecraft] (minecraft.jar) 
    UCHIJA    mcp{9.19} [Minecraft Coder Pack] (minecraft.jar) 
    UCHIJA    FML{8.0.99.99} [Forge Mod Loader] (forge-1.12-14.21.1.2387.jar) 
    UCHIJA    forge{14.21.1.2387} [Minecraft Forge] (forge-1.12-14.21.1.2387.jar) 
    UCHIJA    pixelmon{6.3.1} [Pixelmon] (Pixelmon-1.12.2-6.3.1-universal.jar) 
    Loaded coremods (and transformers): 
    GL info: ' Vendor: 'Intel' Version: '4.5.0 - Build 23.20.16.4982' Renderer: 'Intel(R) HD Graphics 620'
    Launched Version: 1.12-forge1.12-14.21.1.2387
    LWJGL: 2.9.4
    OpenGL: Intel(R) HD Graphics 620 GL version 4.5.0 - Build 23.20.16.4982, 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: English (US)
    Profiler Position: N/A (disabled)
    CPU: 4x Intel(R) Core(TM) i5-7200U CPU @ 2.50GHz

Pixelmon is just broken I wouldn't even bother trying to use it. All you can really do is contact the mod authors and make sure you are on the latest version if they release a fix.

You’re trying to run Pixelmon Reforged, which is for 1.12.2 on Forge for 1.12.0, update to the recommended version of Forge 1.12.2