DaemonUmbra

Please update to a modern version of Minecraft.

Please post the full log as described in my signature.

If you want to play the quote/repeat game

No. Support is not given for versions THAT out of date.

If you've installed the 64-Bit SDK and removed _JAVA_OPTIONS, just point JAVA_HOME at the 64-Bit SDK and you should be good.

Yes, you also need to remove your _JAVA_OPTIONS environment variable. Why do you even have that?

A couple things wrong here: This is not nearly enough to run the compilation process. (x86) means you are likely running 32-Bit Java which will not let you use more than 2G, while the decompilation process is recommended to be run with 3G, which is specified in the gradle.properties that comes with the MDK, it is also recommended to run 64-Bit Java in general if you have a 64-Bit machine.

Do you have a Java folder in /Program Files(x86)/?

Is that the only JDK you have installed?

What's the name of the JDK installer file?

Double check your JAVA_HOME variable, since that's what Gradle gets your JDK from

The only thing I can suggest is re-installing the JDK, because the JVM saying anything higher than 2G "exceeds the maximum representable size" is indicative of the 32-Bit version.

Open a command prompt and type java -version please.

And that was the default install location for the JDK?

Yes that looks alright, and your JAVA_HOME variable points to that?

Can you give me the path of your JDK?

It's one of those mods that extracts a component of itself into mods/1.12.2. You'll need to remove this as well because the component gets loaded and assumes the rest of the mod is there, then crashes when it can't find it.

Are you using Open Terrain Generator or no?

Make sure you're using a 64-bit JDK, not 32-bit.

Please post logs as described in my signature below.

Not according to GitHub

Because you are not building it correctly and you are still using 1.8.X I will help you further when i see your repo updated to a modern version.

I am talking about Forge's Jar-in-Jar system, NOT shadow... The mod you linked uses shadow, not Forge's Jar-in-Jar system

Because Forge's system is superior to just shoving your dependency inside your jar, Forge's system does things to prevent collisions between packed dependencies, as well as archiving them so they only need to be extracted once.

If you want to rely on the legacy system you can use Jar-in-Jar without a "Maven-Artifact" property, though from a code comment "Maven-Artifact" is likely to be required in Forge for 1.13. This means that you can do this without doing anything with meta files FOR NOW.