I have the same problem, I switched straight from 1.16.5 to 1.12.2 modding and i cant launch the program, when i was in 1.16 it launched but now is just not launching and the console is saying this :
[2021-02-08 19:05:05] [INFO ] Natives: D:\forge-1.12.2-14.23.5.2854-mdk\build\natives
[2021-02-08 19:05:05] [INFO ] Main Class: net.minecraft.launchwrapper.Launch
[2021-02-08 19:05:05] [INFO ] Srg2Mcp: D:\forge-1.12.2-14.23.5.2854-mdk\build\createSrgToMcp\output.srg
[2021-02-08 19:05:05] [INFO ] Extra: []
[2021-02-08 19:05:05] [INFO ] Running with arguments: [--version, 1.12, --assetIndex, 1.12, --assetsDir, C:\Users\ROG\.gradle\caches\forge_gradle\assets, --accessToken, {REDACTED}, --userProperties, [], --tweakClass, net.minecraftforge.fml.common.launcher.FMLTweaker]
Exception in thread "main" java.lang.reflect.InvocationTargetException
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:64)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:564)
at net.minecraftforge.legacydev.Main.start(Main.java:86)
at net.minecraftforge.legacydev.MainClient.main(MainClient.java:29)
Caused by: java.lang.ClassCastException: class jdk.internal.loader.ClassLoaders$AppClassLoader cannot be cast to class java.net.URLClassLoader (jdk.internal.loader.ClassLoaders$AppClassLoader and java.net.URLClassLoader are in module java.base of loader 'bootstrap')
at net.minecraft.launchwrapper.Launch.<init>(Launch.java:34)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)
... 6 more
Help me pls