The plugin seems to work however expect your console to be unusable due to the continuous output of errors by this plugin (The error is below on a review almost 3 months ago.)
Really cool plugin, love it. but I don't know how to fix this error.
[02:56:11 ERROR]: Could not pass event NPCRemoveEvent to CitizensPlaceholderAPI v1.0
java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
at java.util.LinkedList.checkElementIndex(Unknown Source) ~[?:1.8.0_271]
at java.util.LinkedList.remove(Unknown Source) ~[?:1.8.0_271]
at me.citizensplaceholderapi.data.NPCDataStorage.deleteNPC(NPCDataStorage.java:136) ~[?:?]
at me.citizensplaceholderapi.CitizensPlaceholderAPI.onNPCRemove(CitizensPlaceholderAPI.java:61) ~[?:?]
at com.destroystokyo.paper.event.executor.asm.generated.GeneratedEventExecutor863.execute(Unknown Source) ~[?:?]
at org.bukkit.plugin.EventExecutor.lambda$create$1(EventExecutor.java:69) ~[patched_1.16.4.jar:git-Paper-291]
at co.aikar.timings.TimedEventExecutor.execute(TimedEventExecutor.java:80) ~[patched_1.16.4.jar:git-Paper-291]
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:70) ~[patched_1.16.4.jar:git-Paper-291]
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:607) ~[patched_1.16.4.jar:git-Paper-291]
at net.citizensnpcs.api.npc.AbstractNPC.destroy(AbstractNPC.java:158) ~[?:?]
at net.citizensnpcs.npc.CitizensNPC.destroy(CitizensNPC.java:113) ~[?:?]
at net.citizensnpcs.trait.HologramTrait.unload(HologramTrait.java:199) ~[?:?]
at net.citizensnpcs.trait.HologramTrait.onDespawn(HologramTrait.java:112) ~[?:?]
at net.citizensnpcs.npc.CitizensNPC.despawn(CitizensNPC.java:100) ~[?:?]
at net.citizensnpcs.api.npc.AbstractNPC.setName(AbstractNPC.java:409) ~[?:?]
at me.citizensplaceholderapi.data.NPCDataStorage.lambda$4(NPCDataStorage.java:183) ~[?:?]
at co.aikar.taskchain.TaskChainTasks$GenericTask.run(TaskChainTasks.java:83) ~[?:?]
at co.aikar.taskchain.TaskChain$TaskHolder.run(TaskChain.java:1312) ~[?:?]
at co.aikar.taskchain.TaskChain$TaskHolder.access$2(TaskChain.java:1285) ~[?:?]
at co.aikar.taskchain.TaskChain.lambda$20(TaskChain.java:1194) ~[?:?]
at org.bukkit.craftbukkit.v1_16_R3.scheduler.CraftTask.run(CraftTask.java:99) ~[patched_1.16.4.jar:git-Paper-291]
at org.bukkit.craftbukkit.v1_16_R3.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:468) ~[patched_1.16.4.jar:git-Paper-291]
at net.minecraft.server.v1_16_R3.MinecraftServer.b(MinecraftServer.java:1297) ~[patched_1.16.4.jar:git-Paper-291]
at net.minecraft.server.v1_16_R3.DedicatedServer.b(DedicatedServer.java:377) ~[patched_1.16.4.jar:git-Paper-291]
at net.minecraft.server.v1_16_R3.MinecraftServer.a(MinecraftServer.java:1212) ~[patched_1.16.4.jar:git-Paper-291]
at net.minecraft.server.v1_16_R3.MinecraftServer.w(MinecraftServer.java:1000) ~[patched_1.16.4.jar:git-Paper-291]
at net.minecraft.server.v1_16_R3.MinecraftServer.lambda$a$0(MinecraftServer.java:177) ~[patched_1.16.4.jar:git-Paper-291]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_271]