How to solve the "io.netty.channel.AbstractChannel$AnnotatedConnectException: Connection refused: no further information" error on Minecraft?

I have the same error on my local network, it looks like a firewall issue on the host/server side. For me, to troubleshoot I used nmap. The LAN game showed up in multiplayer as 10.0.0.21:49299. nmap reported that port from that IP as closed/filtered; i.e. a firewall issue.

To confirm it is a firewall issue, turn off the firewall (momentarily). See if you're able to connect while the firewall is disabled.

It looks like it has something to do with Firewall not allowing traffic to the 1.12.jar file located in C:\Users\user\AppData\Roaming.minecraft\versions(version).

I assume that when you say you have "Altered the Firewall" you mean that you only allowed connections to the Java.exes but NOT the actual 1.12.jar file itself.

Important: When creating the rule it is important to make sure these configurations are set: Set the program path to the 1.12.jar file, and make sure to check "Allow the Connection" when configuring the rule.