Spigot 1.21 does not have CraftInventoryAnvil implement the API class AnvilView, which results in a NoSuchMethod error when attempting to access it from the API:

[18:07:07] [Server thread/ERROR]: Could not pass event PrepareAnvilEvent to EnchantCopy v1.2
org.bukkit.event.EventException: null
    at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:310) ~[spigot-api-1.21-R0.1-SNAPSHOT.jar:?]
    at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:70) ~[spigot-api-1.21-R0.1-SNAPSHOT.jar:?]
    at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:601) ~[spigot-api-1.21-R0.1-SNAPSHOT.jar:?]
    at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:588) ~[spigot-api-1.21-R0.1-SNAPSHOT.jar:?]
    at org.bukkit.craftbukkit.v1_21_R1.event.CraftEventFactory.callPrepareAnvilEvent(CraftEventFactory.java:1550) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.world.inventory.ContainerAnvil.m(ContainerAnvil.java:168) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.world.inventory.ContainerAnvilAbstract.a(ContainerAnvilAbstract.java:112) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.world.inventory.ContainerAnvilAbstract$3.e(ContainerAnvilAbstract.java:103) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.world.InventorySubcontainer.a(InventorySubcontainer.java:204) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.world.inventory.Slot.f(SourceFile:75) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at org.bukkit.craftbukkit.v1_21_R1.inventory.CraftInventoryView.setItem(CraftInventoryView.java:62) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.world.inventory.Container.b(Container.java:480) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.world.inventory.Container.a(Container.java:376) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.server.network.PlayerConnection.a(PlayerConnection.java:2713) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.network.protocol.game.PacketPlayInWindowClick.a(SourceFile:71) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.network.protocol.game.PacketPlayInWindowClick.a(SourceFile:14) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.network.protocol.PlayerConnectionUtils.lambda$ensureRunningOnSameThread$0(PlayerConnectionUtils.java:35) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.server.TickTask.run(SourceFile:18) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.util.thread.IAsyncTaskHandler.d(SourceFile:162) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.util.thread.IAsyncTaskHandlerReentrant.d(SourceFile:23) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.server.MinecraftServer.b(MinecraftServer.java:1270) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.server.MinecraftServer.d(MinecraftServer.java:203) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.util.thread.IAsyncTaskHandler.B(SourceFile:136) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.server.MinecraftServer.bv(MinecraftServer.java:1249) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.server.MinecraftServer.B(MinecraftServer.java:1242) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.util.thread.IAsyncTaskHandler.b(SourceFile:145) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.server.MinecraftServer.b(MinecraftServer.java:1207) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.server.MinecraftServer.v_(MinecraftServer.java:1214) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.server.MinecraftServer.y(MinecraftServer.java:1078) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:318) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at java.base/java.lang.Thread.run(Thread.java:1583) [?:?]
Caused by: java.lang.NoSuchMethodError: 'org.bukkit.inventory.view.AnvilView org.bukkit.event.inventory.PrepareAnvilEvent.getView()'
    at com.voxelbuster.enchantcopy.PluginEventHandler.onAnvilUse(PluginEventHandler.java:61) ~[?:?]
    at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103) ~[?:?]
    at java.base/java.lang.reflect.Method.invoke(Method.java:580) ~[?:?]
    at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:306) ~[spigot-api-1.21-R0.1-SNAPSHOT.jar:?]
    ... 30 more
[18:07:07] [Server thread/ERROR]: Could not pass event PrepareAnvilEvent to EnchantCopy v1.2
org.bukkit.event.EventException: null
    at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:310) ~[spigot-api-1.21-R0.1-SNAPSHOT.jar:?]
    at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:70) ~[spigot-api-1.21-R0.1-SNAPSHOT.jar:?]
    at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:601) ~[spigot-api-1.21-R0.1-SNAPSHOT.jar:?]
    at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:588) ~[spigot-api-1.21-R0.1-SNAPSHOT.jar:?]
    at org.bukkit.craftbukkit.v1_21_R1.event.CraftEventFactory.callPrepareAnvilEvent(CraftEventFactory.java:1550) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.world.inventory.ContainerAnvil.m(ContainerAnvil.java:168) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.world.inventory.ContainerAnvilAbstract.a(ContainerAnvilAbstract.java:112) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.world.inventory.ContainerAnvilAbstract$3.e(ContainerAnvilAbstract.java:103) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.world.inventory.Slot.c(SourceFile:80) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.world.inventory.Slot.f(SourceFile:76) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at org.bukkit.craftbukkit.v1_21_R1.inventory.CraftInventoryView.setItem(CraftInventoryView.java:62) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.world.inventory.Container.b(Container.java:480) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.world.inventory.Container.a(Container.java:376) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.server.network.PlayerConnection.a(PlayerConnection.java:2713) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.network.protocol.game.PacketPlayInWindowClick.a(SourceFile:71) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.network.protocol.game.PacketPlayInWindowClick.a(SourceFile:14) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.network.protocol.PlayerConnectionUtils.lambda$ensureRunningOnSameThread$0(PlayerConnectionUtils.java:35) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.server.TickTask.run(SourceFile:18) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.util.thread.IAsyncTaskHandler.d(SourceFile:162) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.util.thread.IAsyncTaskHandlerReentrant.d(SourceFile:23) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.server.MinecraftServer.b(MinecraftServer.java:1270) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.server.MinecraftServer.d(MinecraftServer.java:203) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.util.thread.IAsyncTaskHandler.B(SourceFile:136) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.server.MinecraftServer.bv(MinecraftServer.java:1249) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.server.MinecraftServer.B(MinecraftServer.java:1242) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.util.thread.IAsyncTaskHandler.b(SourceFile:145) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.server.MinecraftServer.b(MinecraftServer.java:1207) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.server.MinecraftServer.v_(MinecraftServer.java:1214) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.server.MinecraftServer.y(MinecraftServer.java:1078) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:318) ~[spigot-1.21-R0.1-SNAPSHOT.jar:4226-Spigot-146439e-2889b3a]
    at java.base/java.lang.Thread.run(Thread.java:1583) [?:?]
Caused by: java.lang.NoSuchMethodError: 'org.bukkit.inventory.view.AnvilView org.bukkit.event.inventory.PrepareAnvilEvent.getView()'
    at com.voxelbuster.enchantcopy.PluginEventHandler.onAnvilUse(PluginEventHandler.java:61) ~[?:?]
    at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103) ~[?:?]
    at java.base/java.lang.reflect.Method.invoke(Method.java:580) ~[?:?]
    at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:306) ~[spigot-api-1.21-R0.1-SNAPSHOT.jar:?]
    ... 30 more

From spigot JAR (CraftInventoryAnvil.class) [spigot-api-1.21-R0.1-20240807.214924-87.jar]:

package org.bukkit.craftbukkit.v1_21_R1.inventory;

import com.google.common.base.Preconditions;
import net.minecraft.world.IInventory;
import net.minecraft.world.inventory.ContainerAnvil;
import org.bukkit.Location;
import org.bukkit.inventory.AnvilInventory;

public class CraftInventoryAnvil extends CraftResultInventory implements AnvilInventory {
private final Location location;
private final ContainerAnvil container;

...

This forces plugin developers to depend on AnvilView methods from the Spigot server JAR.