Forge doesn't start

hello, I try to make my first step with minecraft forge.  so I download the forge installer 1.12.2 und run the installer.

the "minecraft_server.1.12.2.jar" and the "forge-1.12.2-14.23.5.2855.jar". I start the "minecraft_server.1.12.2.jar"

with java -jar minecraft_server.1.12.2.jar. I give one mod ( "Techguns-Mod-1.12.2.jar") into the directory "mods".

But not all features of the mod runs correct. So I tried to run the "forge-1.12.2-14.23.5.2855.jar" with java -jar forge-1.12.2-14.23.5.2855.jar.

But there the message:

A problem occurred running the Server launcher.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.fml.relauncher.ServerLaunchWrapper.run(ServerLaunchWrapper.java:70)
        at net.minecraftforge.fml.relauncher.ServerLaunchWrapper.main(ServerLaunchWrapper.java:34)
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

appears.

So can someone help ? What I'm doing wrong ?