I keep getting this error when I try to install Forge for 1.12.2 and I've already seen this thread http://www.minecraftforge.net/forum/topic/52285-forge-installation-issues/?tab=comments#comment-255402
But I didn't fully understand what you meant by "replacing [name of the installer] with the complete file name of the installer, including the .jar file extension." this might be me just being stupid but I'm pretty confused