I downloaded the Sky Factory 3 Mod Pack but the launcher will not work. 

 

Java HotSpot(TM) 64-Bit Server VM warning: ignoring option PermSize=256m; support was removed in 8.0
[01:27:36] [main/INFO]: Loading tweak class name net.minecraftforge.fml.common.launcher.FMLTweaker
[01:27:36] [main/ERROR]: Unable to launch
java.lang.ClassNotFoundException: net.minecraftforge.fml.common.launcher.FMLTweaker
    at java.net.URLClassLoader.findClass(URLClassLoader.java:381) ~[?:1.8.0_151]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[?:1.8.0_151]
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:335) ~[?:1.8.0_151]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[?:1.8.0_151]
    at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:106) ~[launchwrapper-1.12.jar:?]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[?:1.8.0_151]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[?:1.8.0_151]
    at java.lang.Class.forName0(Native Method) ~[?:1.8.0_151]
    at java.lang.Class.forName(Class.java:348) ~[?:1.8.0_151]
    at net.minecraft.launchwrapper.Launch.launch(Launch.java:98) [launchwrapper-1.12.jar:?]
    at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.12.jar:?]

 

If you're using Java 9, you need to downgrade to Java 8.

java -version at the commandline will show you what the version is.

I did downgrade to Java 8 but the problem is still there

 

I downloaded Twitch and then I downloaded the sky factory 3 mod pack and it downloaded forge then i pressed play through Twitch and it brought me to the launcher. 

I downloaded Twitch and then I downloaded the sky factory 3 mod pack and it downloaded forge then i pressed play through Twitch and it brought me to the launcher. 

17 minutes ago, diesieben07 said:

How did you install Forge and how are you launching it?