Get your minecraft_server.1.12.2.jar
from the root of your server, download the latest netty 4 full package https://netty.io/downloads.html
Open your server jar with 7zip and navigate to minecraft_server.1.12.2.jar\io\netty\
open the netty package you downloaded and go to netty-4.1.70.Final.tar.gz\netty-4.1.70.Final.tar\netty-4.1.70.Final\jar\all-in-one\netty-all-4.1.70.Final.jar\io\netty\
back in the minecraft server jar delete these 6 folders
now simple copy the same 6 folders from the netty package into the minecraft jar
throw the jar back into the root of your server and you are done!
try reinstalling forge..