I'm having an issue running Forge on MC from 1.12.2, I am on the stock launcher and on the latest version of the launcher, as well I am on the latest version of Java. I have seen to remove Optifine, but I do not have Optifine installed for 1.12.2, but I do have it for 1.14.3. Here's the short version of the error code:
The game crashed whilst initializing game
Error: java.lang.NoClassDefFoundError: net/minecraft/client/renderer/EntityRenderer
The full error code can be found here.