After trying to just install the mod inside a server, I couldn't seem to join in due to an internal error. Only two mods installed are this mod and CustomNPCs, on both server and client.

EDIT: Right, I should probably mention the version. 1.12.2 for Forge.

[19:41:39] [Server thread/ERROR] [FML]: Exception caught during firing event net.minecraftforge.fml.common.gameevent.TickEvent$PlayerTickEvent@76e48480:
java.lang.NoClassDefFoundError: net/minecraft/client/entity/AbstractClientPlayer
at com.github.gamepiaynmo.custommodel.mixin.PlayerStatureHandler.onPlayerUpdate(PlayerStatureHandler.java:54) ~[cpm-1.0.11-forge-mc1.12.2-beta.jar:?]
at net.minecraftforge.fml.common.eventhandler.ASMEventHandler_37_PlayerStatureHandler_onPlayerUpdate_PlayerTickEvent.invoke(.dynamic) ~[?:?]
at net.minecraftforge.fml.common.eventhandler.ASMEventHandler.invoke(ASMEventHandler.java:90) ~[ASMEventHandler.class:?]
at net.minecraftforge.fml.common.eventhandler.EventBus.post(EventBus.java:182) [EventBus.class:?]
at net.minecraftforge.fml.common.FMLCommonHandler.onPlayerPreTick(FMLCommonHandler.java:365) [FMLCommonHandler.class:?]
at net.minecraft.entity.player.EntityPlayer.func_70071_h_(EntityPlayer.java:190) [aed.class:?]
at net.minecraft.entity.player.EntityPlayerMP.func_71127_g(EntityPlayerMP.java:382) [oq.class:?]
at net.minecraft.network.NetHandlerPlayServer.func_73660_a(NetHandlerPlayServer.java:173) [pa.class:?]
at net.minecraftforge.fml.common.network.handshake.NetworkDispatcher$1.func_73660_a(NetworkDispatcher.java:209) [NetworkDispatcher$1.class:?]
at net.minecraft.network.NetworkManager.func_74428_b(NetworkManager.java:285) [gw.class:?]
at net.minecraft.network.NetworkSystem.func_151269_c(NetworkSystem.java:180) [oz.class:?]
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:790) [MinecraftServer.class:?]
at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:397) [nz.class:?]
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:668) [MinecraftServer.class:?]
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:526) [MinecraftServer.class:?]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_151]
Caused by: java.lang.ClassNotFoundException: net.minecraft.client.entity.AbstractClientPlayer
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:191) ~[launchwrapper-1.12.jar:?]
at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[?:1.8.0_151]
at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[?:1.8.0_151]
... 16 more
Caused by: net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerException: Exception in class transformer net.minecraftforge.fml.common.asm.transformers.SideTransformer@23aae55 from coremod FMLCorePlugin
at net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerWrapper.transform(ASMTransformerWrapper.java:260) ~[forge-1.12.2-14.23.5.2847-universal.jar:?]
at net.minecraft.launchwrapper.LaunchClassLoader.runTransformers(LaunchClassLoader.java:279) ~[launchwrapper-1.12.jar:?]
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:176) ~[launchwrapper-1.12.jar:?]
at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[?:1.8.0_151]
at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[?:1.8.0_151]
... 16 more
Caused by: java.lang.RuntimeException: Attempted to load class bua for invalid side SERVER
at net.minecraftforge.fml.common.asm.transformers.SideTransformer.transform(SideTransformer.java:62) ~[forge-1.12.2-14.23.5.2847-universal.jar:?]
at net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerWrapper.transform(ASMTransformerWrapper.java:256) ~[forge-1.12.2-14.23.5.2847-universal.jar:?]
at net.minecraft.launchwrapper.LaunchClassLoader.runTransformers(LaunchClassLoader.java:279) ~[launchwrapper-1.12.jar:?]
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:176) ~[launchwrapper-1.12.jar:?]
at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[?:1.8.0_151]
at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[?:1.8.0_151]
... 16 more
[19:41:39] [Server thread/ERROR] [FML]: Index: 3 Listeners:
[19:41:39] [Server thread/ERROR] [FML]: 0: NORMAL
[19:41:39] [Server thread/ERROR] [FML]: 1: ASM: noppes.npcs.ServerTickHandler@1d00b68f onServerTick(Lnet/minecraftforge/fml/common/gameevent/TickEvent$PlayerTickEvent;)V
[19:41:39] [Server thread/ERROR] [FML]: 2: ASM: class com.github.gamepiaynmo.custommodel.server.CustomModel onPlayerUpdate(Lnet/minecraftforge/fml/common/gameevent/TickEvent$PlayerTickEvent;)V
[19:41:39] [Server thread/ERROR] [FML]: 3: ASM: class com.github.gamepiaynmo.custommodel.mixin.PlayerStatureHandler onPlayerUpdate(Lnet/minecraftforge/fml/common/gameevent/TickEvent$PlayerTickEvent;)V
[19:41:39] [Server thread/ERROR] [FML]: 4: ASM: noppes.npcs.ScriptPlayerEventHandler$ForgeEventHandler@156e7bd1 forgeEntity(Lnet/minecraftforge/fml/common/eventhandler/Event;)V
[19:41:39] [Server thread/ERROR] [FML]: 5: ASM: noppes.npcs.ScriptPlayerEventHandler@278e71c1 onServerTick(Lnet/minecraftforge/fml/common/gameevent/TickEvent$PlayerTickEvent;)V
[19:41:39] [Server thread/WARN] [minecraft/NetworkSystem]: Failed to handle packet for /76.95.12.164:49968
net.minecraft.util.ReportedException: Ticking player
at net.minecraft.entity.player.EntityPlayerMP.func_71127_g(EntityPlayerMP.java:459) ~[oq.class:?]
at net.minecraft.network.NetHandlerPlayServer.func_73660_a(NetHandlerPlayServer.java:173) ~[pa.class:?]
at net.minecraftforge.fml.common.network.handshake.NetworkDispatcher$1.func_73660_a(NetworkDispatcher.java:209) ~[NetworkDispatcher$1.class:?]
at net.minecraft.network.NetworkManager.func_74428_b(NetworkManager.java:285) ~[gw.class:?]
at net.minecraft.network.NetworkSystem.func_151269_c(NetworkSystem.java:180) [oz.class:?]
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:790) [MinecraftServer.class:?]
at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:397) [nz.class:?]
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:668) [MinecraftServer.class:?]
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:526) [MinecraftServer.class:?]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_151]
Caused by: java.lang.NoClassDefFoundError: net/minecraft/client/entity/AbstractClientPlayer
at com.github.gamepiaynmo.custommodel.mixin.PlayerStatureHandler.onPlayerUpdate(PlayerStatureHandler.java:54) ~[cpm-1.0.11-forge-mc1.12.2-beta.jar:?]
at net.minecraftforge.fml.common.eventhandler.ASMEventHandler_37_PlayerStatureHandler_onPlayerUpdate_PlayerTickEvent.invoke(.dynamic) ~[?:?]
at net.minecraftforge.fml.common.eventhandler.ASMEventHandler.invoke(ASMEventHandler.java:90) ~[ASMEventHandler.class:?]
at net.minecraftforge.fml.common.eventhandler.EventBus.post(EventBus.java:182) ~[EventBus.class:?]
at net.minecraftforge.fml.common.FMLCommonHandler.onPlayerPreTick(FMLCommonHandler.java:365) ~[FMLCommonHandler.class:?]
at net.minecraft.entity.player.EntityPlayer.func_70071_h_(EntityPlayer.java:190) ~[aed.class:?]
at net.minecraft.entity.player.EntityPlayerMP.func_71127_g(EntityPlayerMP.java:382) ~[oq.class:?]
... 9 more
Caused by: java.lang.ClassNotFoundException: net.minecraft.client.entity.AbstractClientPlayer
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:191) ~[launchwrapper-1.12.jar:?]
at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[?:1.8.0_151]
at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[?:1.8.0_151]
at com.github.gamepiaynmo.custommodel.mixin.PlayerStatureHandler.onPlayerUpdate(PlayerStatureHandler.java:54) ~[cpm-1.0.11-forge-mc1.12.2-beta.jar:?]
at net.minecraftforge.fml.common.eventhandler.ASMEventHandler_37_PlayerStatureHandler_onPlayerUpdate_PlayerTickEvent.invoke(.dynamic) ~[?:?]
at net.minecraftforge.fml.common.eventhandler.ASMEventHandler.invoke(ASMEventHandler.java:90) ~[ASMEventHandler.class:?]
at net.minecraftforge.fml.common.eventhandler.EventBus.post(EventBus.java:182) ~[EventBus.class:?]
at net.minecraftforge.fml.common.FMLCommonHandler.onPlayerPreTick(FMLCommonHandler.java:365) ~[FMLCommonHandler.class:?]
at net.minecraft.entity.player.EntityPlayer.func_70071_h_(EntityPlayer.java:190) ~[aed.class:?]
at net.minecraft.entity.player.EntityPlayerMP.func_71127_g(EntityPlayerMP.java:382) ~[oq.class:?]
... 9 more
Caused by: net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerException: Exception in class transformer net.minecraftforge.fml.common.asm.transformers.SideTransformer@23aae55 from coremod FMLCorePlugin
at net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerWrapper.transform(ASMTransformerWrapper.java:260) ~[forge-1.12.2-14.23.5.2847-universal.jar:?]
at net.minecraft.launchwrapper.LaunchClassLoader.runTransformers(LaunchClassLoader.java:279) ~[launchwrapper-1.12.jar:?]
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:176) ~[launchwrapper-1.12.jar:?]
at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[?:1.8.0_151]
at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[?:1.8.0_151]
at com.github.gamepiaynmo.custommodel.mixin.PlayerStatureHandler.onPlayerUpdate(PlayerStatureHandler.java:54) ~[cpm-1.0.11-forge-mc1.12.2-beta.jar:?]
at net.minecraftforge.fml.common.eventhandler.ASMEventHandler_37_PlayerStatureHandler_onPlayerUpdate_PlayerTickEvent.invoke(.dynamic) ~[?:?]
at net.minecraftforge.fml.common.eventhandler.ASMEventHandler.invoke(ASMEventHandler.java:90) ~[ASMEventHandler.class:?]
at net.minecraftforge.fml.common.eventhandler.EventBus.post(EventBus.java:182) ~[EventBus.class:?]
at net.minecraftforge.fml.common.FMLCommonHandler.onPlayerPreTick(FMLCommonHandler.java:365) ~[FMLCommonHandler.class:?]
at net.minecraft.entity.player.EntityPlayer.func_70071_h_(EntityPlayer.java:190) ~[aed.class:?]
at net.minecraft.entity.player.EntityPlayerMP.func_71127_g(EntityPlayerMP.java:382) ~[oq.class:?]
... 9 more
Caused by: java.lang.RuntimeException: Attempted to load class bua for invalid side SERVER
at net.minecraftforge.fml.common.asm.transformers.SideTransformer.transform(SideTransformer.java:62) ~[forge-1.12.2-14.23.5.2847-universal.jar:?]
at net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerWrapper.transform(ASMTransformerWrapper.java:256) ~[forge-1.12.2-14.23.5.2847-universal.jar:?]
at net.minecraft.launchwrapper.LaunchClassLoader.runTransformers(LaunchClassLoader.java:279) ~[launchwrapper-1.12.jar:?]
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:176) ~[launchwrapper-1.12.jar:?]
at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[?:1.8.0_151]
at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[?:1.8.0_151]
at com.github.gamepiaynmo.custommodel.mixin.PlayerStatureHandler.onPlayerUpdate(PlayerStatureHandler.java:54) ~[cpm-1.0.11-forge-mc1.12.2-beta.jar:?]
at net.minecraftforge.fml.common.eventhandler.ASMEventHandler_37_PlayerStatureHandler_onPlayerUpdate_PlayerTickEvent.invoke(.dynamic) ~[?:?]
at net.minecraftforge.fml.common.eventhandler.ASMEventHandler.invoke(ASMEventHandler.java:90) ~[ASMEventHandler.class:?]
at net.minecraftforge.fml.common.eventhandler.EventBus.post(EventBus.java:182) ~[EventBus.class:?]
at net.minecraftforge.fml.common.FMLCommonHandler.onPlayerPreTick(FMLCommonHandler.java:365) ~[FMLCommonHandler.class:?]
at net.minecraft.entity.player.EntityPlayer.func_70071_h_(EntityPlayer.java:190) ~[aed.class:?]
at net.minecraft.entity.player.EntityPlayerMP.func_71127_g(EntityPlayerMP.java:382) ~[oq.class:?]
... 9 more

This bug is fixed in the latest version.