DaemonUmbra

Could you provide the full error please? There should be a bit above the actual failure notice.

It looks to be broken, try removing it

Did you make the "Default 1.12" resource pack?

Please provide your debug.log file using one of the paste sites in my signature

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

https://github.com/MinecraftForge/MinecraftForge/pull/6455

I'm pretty sure this was fixed early on, what Forge version are you running?

Looks like your world is corrupted

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

Try running JarFix once, then try again

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

Please keep this forum in English

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

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

1.13 is no longer supported on this forum due to its age. Please update to a modern version of Minecraft to receive support.

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

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

Threads merged. What version are you modding?

Do you know Java?

What thread are you following?

Sounding a bit like a modded client on a vanilla server

You upload it to one of the sites listed in my signature and post the link it gives you

Please provide debug.log using one of the paste sites listed in my signature below

I feel like you answered the question before you asked it?