I've made my own custom mod, made for 1.12.2 and have been trying to add it to a private server, it's the ONLY mod in the server besides forge, and every time I go back and re-think the code it gives me the exact same error.

The mod works just fine in singleplayer, the server's crash log:

 

---- Minecraft Crash Report ----
// Oops.

Time: 1/5/19 9:05 PM
Description: Exception in server tick loop

net.minecraftforge.fml.common.LoaderExceptionModCrash: Caught exception from Vanilla Spells 1.12.2 (vanillaspellsmod)
Caused by: java.lang.NoClassDefFoundError: net/minecraft/client/entity/EntityPlayerSP
    at java.lang.Class.getDeclaredConstructors0(Native Method)
    at java.lang.Class.privateGetDeclaredConstructors(Unknown Source)
    at java.lang.Class.getConstructor0(Unknown Source)
    at java.lang.Class.newInstance(Unknown Source)
    at net.minecraftforge.fml.common.network.simpleimpl.SimpleNetworkWrapper.instantiate(SimpleNetworkWrapper.java:166)
    at net.minecraftforge.fml.common.network.simpleimpl.SimpleNetworkWrapper.registerMessage(SimpleNetworkWrapper.java:159)
    at network.PacketHandler.registerMessages(PacketHandler.java:24)
    at com.vanillaspellsmod.mainRegistry.VanillaSpells.preInit(VanillaSpells.java:52)
    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 net.minecraftforge.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:624)
    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$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:116)
    at com.google.common.eventbus.EventBus.post(EventBus.java:217)
    at net.minecraftforge.fml.common.LoadController.distributeStateMessage(LoadController.java:136)
    at net.minecraftforge.fml.common.Loader.preinitializeMods(Loader.java:627)
    at net.minecraftforge.fml.server.FMLServerHandler.beginServerLoading(FMLServerHandler.java:99)
    at net.minecraftforge.fml.common.FMLCommonHandler.onServerStart(FMLCommonHandler.java:333)
    at net.minecraft.server.dedicated.DedicatedServer.func_71197_b(DedicatedServer.java:125)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:486)
    at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: net.minecraft.client.entity.EntityPlayerSP
    at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:191)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    ... 44 more
Caused by: net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerException: Exception in class transformer net.minecraftforge.fml.common.asm.transformers.SideTransformer@5ab9b447 from coremod FMLCorePlugin
    at net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerWrapper.transform(ASMTransformerWrapper.java:260)
    at net.minecraft.launchwrapper.LaunchClassLoader.runTransformers(LaunchClassLoader.java:279)
    at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:176)
    ... 46 more
Caused by: java.lang.RuntimeException: Attempted to load class bud for invalid side SERVER
    at net.minecraftforge.fml.common.asm.transformers.SideTransformer.transform(SideTransformer.java:62)
    at net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerWrapper.transform(ASMTransformerWrapper.java:256)
    ... 48 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_121, Oracle Corporation
    Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
    Memory: 889225752 bytes (848 MB) / 1054867456 bytes (1006 MB) up to 3806855168 bytes (3630 MB)
    JVM Flags: 0 total; 
    IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
    FML: MCP 9.42 Powered by Forge 14.23.5.2795 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

    | State | ID               | Version      | Source                                  | Signature                                |
    |:----- |:---------------- |:------------ |:--------------------------------------- |:---------------------------------------- |
    | LCH   | minecraft        | 1.12.2       | minecraft.jar                           | None                                     |
    | LCH   | mcp              | 9.42         | minecraft.jar                           | None                                     |
    | LCH   | FML              | 8.0.99.99    | forge-1.12.2-14.23.5.2795-universal.jar | e3c3d50c7c986df74c645c0ac54639741c90a557 |
    | LCH   | forge            | 14.23.5.2795 | forge-1.12.2-14.23.5.2795-universal.jar | e3c3d50c7c986df74c645c0ac54639741c90a557 |
    | LCE   | vanillaspellsmod | 0.1          | VanillaSpells0.1(1.12.2).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)

It points to a class in my self-made mod, what I think is really important is the part about the packethandler class:

 

package network;

import net.minecraftforge.fml.common.network.NetworkRegistry;
import net.minecraftforge.fml.common.network.simpleimpl.SimpleNetworkWrapper;
import net.minecraftforge.fml.relauncher.Side;

public class PacketHandler {

    public static SimpleNetworkWrapper INSTANCE;
    
    /*private static int ID = 0;
    
    private static int nextID() {
        return ID++;
    }*/
    
    public static void registerMessages(String channelName) {
        INSTANCE = NetworkRegistry.INSTANCE.newSimpleChannel(channelName);
        int idx = 0;
    //client -> server
    INSTANCE.registerMessage(PacketToServerUpdateCapability.Handler.class, PacketToServerUpdateCapability.class, idx, Side.SERVER);
        idx++;
    //server -> client
    INSTANCE.registerMessage(PacketToClientUpdatedExperience.Handler.class, PacketToClientUpdatedExperience.class, idx, Side.CLIENT);
        idx++;
    INSTANCE.registerMessage(PacketToClientUpdateCapValue.Handler.class, PacketToClientUpdateCapValue.class, idx, Side.CLIENT);
    
    }

    
    
}
 

Fixed this problem, on to the next then.