So I'm trying to start a Minecraft server of the ATM9 modpack - downloaded the file from curseforge, all seemed fine, until I tried to start the server I'm getting an error. I THINK it has somtething to do with forge not installing properly but I'm not sure:|
 

Host: files.minecraftforge.net [51.79.83.165]
Host: maven.minecraftforge.net [51.79.83.165]
Host: libraries.minecraft.net [13.107.213.45, 13.107.246.45]
Host: launchermeta.mojang.com [13.107.213.45, 13.107.246.45]
Host: piston-meta.mojang.com [13.107.246.45, 13.107.213.45]
java.net.UnknownHostException: No such host is known (authserver.mojang.com)
        at java.base/java.net.Inet6AddressImpl.lookupAllHostAddr(Native Method)
        at java.base/java.net.InetAddress$PlatformNameService.lookupAllHostAddr(InetAddress.java:933)
        at java.base/java.net.InetAddress.getAddressesFromNameService(InetAddress.java:1543)
        at java.base/java.net.InetAddress$NameServiceAddresses.get(InetAddress.java:852)
        at java.base/java.net.InetAddress.getAllByName0(InetAddress.java:1532)
        at java.base/java.net.InetAddress.getAllByName(InetAddress.java:1384)
        at java.base/java.net.InetAddress.getAllByName(InetAddress.java:1305)
        at net.minecraftforge.installer.DownloadUtils.getIps(DownloadUtils.java:290)
        at net.minecraftforge.installer.SimpleInstaller.main(SimpleInstaller.java:120)
Exception in thread "main"
java.lang.NullPointerException: Cannot invoke "java.util.List.stream()" because the return value of "net.minecraftforge.installer.DownloadUtils.getIps(String)" is null
        at net.minecraftforge.installer.SimpleInstaller.main(SimpleInstaller.java:120)
Error: could not open `libraries/net/minecraftforge/forge/1.20.1-47.1.3/win_args.txt'
Restarting automatically in 10 seconds (press Ctrl + C to cancel)

Any ideas as to how I can fix this?

Came across this while troubleshooting myself, you're using an old version of the forge installer with an old domain for the files. Download a newer version of the installer file from this page: https://files.minecraftforge.net/net/minecraftforge/forge/ and it should work. Worked for me at least.