Novârch

Do the above, the mountable thing was a passing comment.

Post the full debug.log, the above tells me nothing. From what I know, that means nothing, it's just a closing message. Is your entity mountable? This will always be null if it isn't, not important.

Stairs?

I've been having some issues with running my mod in multiplayer lately, I've tried to diagnose the problem myself but to no avail. The crash in the debug.log doesn't even mention my mod, just says there was an error executing a task on the client, then throws an IndexOutOfBoundsException, I get these all the time in singleplayer and they don't do anything, in singleplayer they clearly come from this line, because the the value I'm writing is null on startup, though that isn't relevant as the crash happens even when that entity isn't present. Does anyone know whats causing these crashes? I'd really appreciate some help.

Try overriding Entity#createSpawnPacket in your entity and return NetworkHooks#getEntitySpawningPacket from it.

Please don't tell me you copied it word for word.

An ItemEntity, Minecraft (or Forge, idk) makes one for your items automatically.

Seems fine for the most part, you should also make this class client only, @OnlyIn does literally nothing for modders, it's just an ugly ASM hack for Forge and Minecraft internal use. Edit: Well...upon further inspection I think you're code won't function on the server at all, you can't just store static variables in an event handler, remember, there is only one instance of your mod on the server, so every player is going to try to share those variables, breaking everything.

Please post your code, preferably as a git repository.

No one really uses or understands data fixers, the error is really unimportant anyway.

1.12 is no longer supported on this forum. Please update to a modern version of Minecraft to receive support,

en_us.json, that's one, look through the resource for the rest. Look at every single file you've created, and fix it.

That is literally your error, some of your jsons have a modid with a capital letter in them, your modid must be all lowercase.

Well make one, it's your project, it's not even important whether you have one.

BipedModel#isSneak. Make sure to offset it like in PlayerRenderer line 67. Oops, that's for crouching, look at LivingRenderer line 55 and below for sitting,

Read Draco's comment, the code is completely different in 1.15, try to make something on your own and come back here if you fail.

Do you think or do you know? Post updated 1.15 code.

1.12 is no longer supported on this forum. Please update to a modern version of Minecraft to receive support.

Take a look at the usages of GameSettings#keyBindInventory (line 137).

Have you checked instanceof, whether you're on the server with World#isRemote? These are potential issues.

Post code, preferably as a git repo. I can't say anything without seeing the json and it's position.

Maybe, but it is still recommended you mod normally and in a way your mod can work with multiple players. Nope, advancements are server-side, you cannot modify them from the client. Why? Because the client is a liar.

Minecraft#player is a ClientPlayerEntity, your NP comes from you trying to get it's server. That is not a good thing to do, unless your mod is client-side, you should always support multiplayer. If your mod is client-side, I'm pretty sure you can't do this, the server doesn't trust the client. Where are you running this code? Context is important.

Take a look at AdvancementCommand , it seems helpful.

You never register your capability in FMLCommonSetupEvent using CapabilityManager#register.