An illegal reflective access operation has occurred when trying to run Minecraft from Eclipse

I want to start modding Minecraft, but I am unable to launch it from Eclipse. Here is the error message I recieve when I try to use Run Configurations with net.minecraftforge.legacydev.MainClient:

[2020-10-18 19:22:51] [INFO   ] Natives: C:\Users\MyPC\Documents\Projects\Forge-MDK\build\natives 
WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by net.minecraftforge.legacydev.MainClient (file:/C:/Users/MyPC/.gradle/caches/modules-2/files-2.1/net.minecraftforge/legacydev/0.2.3.1/f2cd4a8c9f09faf1c1e4a9a4c65df36e634f95db/legacydev-0.2.3.1-fatjar.jar) to method java.lang.ClassLoader.initializePath(java.lang.String)
WARNING: Please consider reporting this to the maintainers of net.minecraftforge.legacydev.MainClient
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release
[2020-10-18 19:22:51] [INFO   ] Main Class: net.minecraft.launchwrapper.Launch 
[2020-10-18 19:22:51] [INFO   ] Srg2Mcp: C:\Users\MyPC\Documents\Projects\Forge-MDK\build\createSrgToMcp\output.srg 
[2020-10-18 19:22:51] [INFO   ] Extra: [] 
[2020-10-18 19:22:51] [INFO   ] Running with arguments: [--version, 1.16.2, --assetIndex, 1.12, --assetsDir, C:\Users\MyPC\.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:62)
	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

I've checked out similar questions to mine before, and the general consensus was that the newest version of Java was not supported by Minecraft, and I would have to use Java 8 (or 1.8). So, I changed my Eclipse preferences to have my execution environment set to Java Platform, Standard Edition 8.0. I also downloaded the JDK with version openjdk version "1.8.0_265" and set the compiler preference to 1.8. None of these methods have resolved this issue. I know that this issue should not be possible if I am using Java 8, so what am I forgetting about?

Thanks in advance!

I just solved this issue for myself. I had to add jre1.8.0_261 to "Java > Installed JREs", then select "Java > Installed JREs > Execution Environments" and make sure the dropdown shows "CDC-1.0/Foundation-1.0(jre1.8.0_261)".

i have the same problem but idk how to solve it, at Java>Installed JREs i have

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C:\Users\Paulo\.p2\pool\plugins\org.eclipse.justj.openjdk.hotspot.jre.full.win32.x86_64_14.0.2.v20200815-0932\jre

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and at Java> Installed JREs>Execution Environments i have
CDC-1.0/Foundation-1.0
OSGi/Minimum-1.0
CDC-1.1/Foundation-1.1
JRE-1.1
OSGi/Minimum-1.1
J2SE-1.2
OSGi/Minimum-1.2
J2SE-1.3
J2SE-1.4
J2SE-1.5
JavaSE-1.6
JavaSE-1.7
JavaSE-1.8
JavaSE-9
JavaSE-10
JavaSE-11
JavaSE-12
JavaSE-13
JavaSE-14

and at the table none has CDC 1.0/ Foundation 1.0(JRE 1.8.0 261)