My server keeps Crashing and I don't know why

[Neon_KnightYT]

I'm new to starting my own server for myself and friends and I'm trying to start a pixelmon Server. Someone please help.

---- Minecraft Crash Report ----
// Shall we play a game?

Time: 3/10/21 2:37 PM
Description: Exception in server tick loop

java.lang.NoClassDefFoundError: net/minecraft/client/gui/GuiScreen
    at de.keksuccino.konkrete.Konkrete.<init>(Konkrete.java:23)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
    at java.lang.reflect.Constructor.newInstance(Unknown Source)
    at java.lang.Class.newInstance(Unknown Source)
    at net.minecraftforge.fml.common.ILanguageAdapter$JavaAdapter.getNewInstance(ILanguageAdapter.java:191)
    at net.minecraftforge.fml.common.FMLModContainer.constructMod(FMLModContainer.java:599)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at com.google.common.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:91)
    at com.google.common.eventbus.Subscriber$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$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:116)
    at com.google.common.eventbus.EventBus.post(EventBus.java:217)
    at net.minecraftforge.fml.common.LoadController.sendEventToModContainer(LoadController.java:219)
    at net.minecraftforge.fml.common.LoadController.propogateStateMessage(LoadController.java:197)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at com.google.common.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:91)
    at com.google.common.eventbus.Subscriber$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$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:116)
    at com.google.common.eventbus.EventBus.post(EventBus.java:217)
    at net.minecraftforge.fml.common.LoadController.distributeStateMessage(LoadController.java:136)
    at net.minecraftforge.fml.common.Loader.loadMods(Loader.java:595)
    at net.minecraftforge.fml.server.FMLServerHandler.beginServerLoading(FMLServerHandler.java:98)
    at net.minecraftforge.fml.common.FMLCommonHandler.onServerStart(FMLCommonHandler.java:333)
    at net.minecraft.server.dedicated.DedicatedServer.func_71197_b(DedicatedServer.java:125)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:486)
    at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: net.minecraft.client.gui.GuiScreen
    at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:191)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    ... 39 more
Caused by: net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerException: Exception in class transformer net.minecraftforge.fml.common.asm.transformers.SideTransformer@4248ed58 from coremod FMLCorePlugin
    at net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerWrapper.transform(ASMTransformerWrapper.java:260)
    at net.minecraft.launchwrapper.LaunchClassLoader.runTransformers(LaunchClassLoader.java:279)
    at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:176)
    ... 41 more
Caused by: java.lang.RuntimeException: Attempted to load class blk for invalid side SERVER
    at net.minecraftforge.fml.common.asm.transformers.SideTransformer.transform(SideTransformer.java:62)
    at net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerWrapper.transform(ASMTransformerWrapper.java:256)
    ... 43 more

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

-- System Details --
Details:
    Minecraft Version: 1.12.2
    Operating System: Windows 10 (amd64) version 10.0
    Java Version: 1.8.0_301, Oracle Corporation
    Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
    Memory: 8407526424 bytes (8018 MB) / 8589934592 bytes (8192 MB) up to 8589934592 bytes (8192 MB)
    JVM Flags: 11 total; -Xmx8G -Xms8G -XX:+UseG1GC -XX:+UnlockExperimentalVMOptions -XX:MaxGCPauseMillis=100 -XX:+DisableExplicitGC -XX:TargetSurvivorRatio=90 -XX:G1NewSizePercent=50 -XX:G1MaxNewSizePercent=80 -XX:G1MixedGCLiveThresholdPercent=50 -XX:+AlwaysPreTouch
    IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
    FML: MCP 9.42 Powered by Forge 14.23.5.2854 13 mods loaded, 13 mods active
    States: 'U' = Unloaded 'L' = Loaded 'C' = Constructed 'H' = Pre-initialized 'I' = Initialized 'J' = Post-initialized 'A' = Available 'D' = Disabled 'E' = Errored

    | State | ID             | Version      | Source                                        | Signature                                |
    |:----- |:-------------- |:------------ |:--------------------------------------------- |:---------------------------------------- |
    | LC    | minecraft      | 1.12.2       | minecraft.jar                                 | None                                     |
    | LC    | mcp            | 9.42         | minecraft.jar                                 | None                                     |
    | LC    | FML            | 8.0.99.99    | forgeserver.jar                               | e3c3d50c7c986df74c645c0ac54639741c90a557 |
    | LC    | forge          | 14.23.5.2854 | forgeserver.jar                               | e3c3d50c7c986df74c645c0ac54639741c90a557 |
    | LC    | biomesoplenty  | 7.0.1.2444   | BiomesOPlenty-1.12.2-7.0.1.2444-universal.jar | None                                     |
    | LC    | bop_patch      | 1.0          | BOP-Patch-1.0.jar                             | None                                     |
    | L     | konkrete       | 1.0.3        | konkrete_1.0.3_MC_1.12-1.12.2.jar             | None                                     |
    | L     | fancymenu      | 1.5.2        | fancymenu_1.5.2_MC_1.12-1.12.2.jar            | None                                     |
    | L     | pixelmon       | 8.3.0        | Pixelmon-1.12.2-8.3.0-universal.jar           | None                                     |
    | L     | gameshark      | 6.0.4        | Gameshark-1.12.2-6.0.4-universal.jar          | None                                     |
    | L     | journeymap     | 1.12.2-5.7.1 | journeymap-1.12.2-5.7.1.jar                   | None                                     |
    | L     | naturescompass | 1.8.5        | NaturesCompass-1.12.2-1.8.5.jar               | None                                     |
    | L     | pixelextras    | 2.5.12       | PixelExtras-1.12.2-2.5.12-universal.jar       | None                                     |

    Loaded coremods (and transformers): 
    Profiler Position: N/A (disabled)
    Is Modded: Definitely; Server brand changed to 'fml,forge'
    Type: Dedicated Server (map_server.txt)

[Luis_ST]

1.12 is no longer supported on this forum.
Please update to a modern version of Minecraft to receive support.

[Neon_KnightYT]

ok