Like another user here on the forums, I too have come up with a problem regarding this update of Forge.
While browsing through the comments and the original post, I can confirm I am running Java 8 (not 9), and have the same system requirements to be able to run Forge.
I have tried to uninstall Forge and reinstall it, but that did not help. If you want to know, I am also running 1.11.2 Forge, and it runs completely fine. However, I go to use 1.12 and it does not want to start. I can confirm again that it has successfully installed, but it is not starting when I choose "Play". I am using the Minecraft Launcher 1.6.67 (the old launcher) on a Windows 10 laptop. I have done the basic tests to see what all I have.
1) Correct Java version: Java 8.
2) Have I used 1.12.2 before? Answer: Yes.
3) Have I used Forge before? Answer: Yes.
4) Anything else you are using besides Forge? Answer: Yes. I am using OptiFine, in the mods folder, along with WorldEdit.
The output I get in the launcher is the following (a bit long):
"
If anyone can help me with this, that will be greatly appreciated.
Side Note: I also run a Mac OS High Sierra MacBook Air, and I can run Minecraft 1.12.2 Forge just fine. It does not, however, work on my Windows 10 laptop for some reason. Please help me!
-- TobyMinceraft