The Model-Engine plugin requires the use of the leather_horse_armor material
But the automatic download of the material package will overwrite the leather_horse_armor and make the Model-Engine plugin unusable.

Try /mhelp modelengine or ask this on Discord, the bot has answers. You need to merge the RPs, there are detailed instructions.

Use /getrp download or follow the instructions to merge and then host your own RP.

There will be no spell list on the experience bar when you use the material pack on your own after not using the automatic download

Error using /getrp download command
[00:13:38] [Server thread/INFO]: tang_guo issued server command: /getrp download
[00:13:38] [Server thread/ERROR]: null
org.bukkit.command.CommandException: Unhandled exception executing command 'getrp' in plugin Magic v10.1.1-addec75
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:47) ~[patched_1.17.1.jar:git-Airplane-55]
at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:172) ~[patched_1.17.1.jar:git-Airplane-55]
at org.bukkit.craftbukkit.v1_17_R1.CraftServer.dispatchCommand(CraftServer.java:869) ~[patched_1.17.1.jar:git-Airplane-55]
at net.minecraft.server.network.ServerGamePacketListenerImpl.handleCommand(ServerGamePacketListenerImpl.java:2330) ~[app:?]
at net.minecraft.server.network.ServerGamePacketListenerImpl.handleChat(ServerGamePacketListenerImpl.java:2141) ~[app:?]
at net.minecraft.server.network.ServerGamePacketListenerImpl.handleChat(ServerGamePacketListenerImpl.java:2122) ~[app:?]
at net.minecraft.network.protocol.game.ServerboundChatPacket.handle(ServerboundChatPacket.java:46) ~[app:?]
at net.minecraft.network.protocol.game.ServerboundChatPacket.handle(ServerboundChatPacket.java:6) ~[app:?]
at net.minecraft.network.protocol.PacketUtils.lambda$ensureRunningOnSameThread$1(PacketUtils.java:56) ~[app:?]
at net.minecraft.server.TickTask.run(TickTask.java:18) ~[patched_1.17.1.jar:git-Airplane-55]
at net.minecraft.util.thread.BlockableEventLoop.doRunTask(BlockableEventLoop.java:149) ~[app:?]
at net.minecraft.util.thread.ReentrantBlockableEventLoop.doRunTask(ReentrantBlockableEventLoop.java:23) ~[app:?]
at net.minecraft.server.MinecraftServer.doRunTask(MinecraftServer.java:1438) ~[patched_1.17.1.jar:git-Airplane-55]
at net.minecraft.server.MinecraftServer.shouldRun(MinecraftServer.java:192) ~[patched_1.17.1.jar:git-Airplane-55]
at net.minecraft.util.thread.BlockableEventLoop.pollTask(BlockableEventLoop.java:122) ~[app:?]
at net.minecraft.server.MinecraftServer.pollTaskInternal(MinecraftServer.java:1416) ~[patched_1.17.1.jar:git-Airplane-55]
at net.minecraft.server.MinecraftServer.pollTask(MinecraftServer.java:1409) ~[patched_1.17.1.jar:git-Airplane-55]
at net.minecraft.util.thread.BlockableEventLoop.managedBlock(BlockableEventLoop.java:132) ~[app:?]
at net.minecraft.server.MinecraftServer.waitUntilNextTick(MinecraftServer.java:1387) ~[patched_1.17.1.jar:git-Airplane-55]
at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1298) ~[patched_1.17.1.jar:git-Airplane-55]
at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:321) ~[patched_1.17.1.jar:git-Airplane-55]
at java.lang.Thread.run(Thread.java:831) ~[?:?]
Caused by: java.lang.NullPointerException: Cannot invoke "java.lang.CharSequence.toString()" because "replacement" is null
at java.lang.String.replace(String.java:2159) ~[?:?]
at com.elmakers.mine.bukkit.magic.command.RPCommandExecutor.onCommand(RPCommandExecutor.java:69) ~[[魔法]Magic-10.1.1.jar:?]
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:45) ~[patched_1.17.1.jar:git-Airplane-55]
... 21 more

What does /version magic say?

Thanks- there is a bug there, I guess you have turned off the RP?

For now you have three options:

1. Follow the instructions to merge your RP with the Magic RP, host it and set Magic to give you that RP
2. Leave the magic config alone, all of your players have to use /getrp download which will make the plugin skip sending them the RP
3. Wait for a fix

I fixed the error you got and made it so /getrp download can work even with the RP disabled.

I've also added a new option for config.yml, assume_resource_pack_downloaded: true

If you want the changes, you can get the dev build from here:
https://jenkins.elmakers.com/job/MagicPlugin/4058/artifact/Magic/target/Magic-10.2-SNAPSHOT.jar