Hello. 

Quick question. Does someone know what this error means when I start my 1.12.2 Forge server and how I can fix it?

[Pterodactyl Daemon]: Checking server disk space usage, this could take a few seconds...
[Pterodactyl Daemon]: Updating process configuration files...
[Pterodactyl Daemon]: Ensuring file permissions are set correctly, this could take a few seconds...
container@pterodactyl~ Server marked as starting...
[Pterodactyl Daemon]: Pulling Docker container image, this could take a few minutes to complete...
[Pterodactyl Daemon]: Finished pulling Docker container image
openjdk version "16.0.2" 2021-07-20
OpenJDK Runtime Environment (build 16.0.2+7-67)
OpenJDK 64-Bit Server VM (build 16.0.2+7-67, mixed mode, sharing)
:/home/container$ java -Dlog4j2.formatMsgNoLookups=true -Xms128M -Xmx2548M -Dterminal.jline=false -Dcom.mojang.eula.agree=true -Dterminal.ansi=true -jar server.jar
JVM info: Oracle Corporation - 16.0.2 - 16.0.2+7-67
java.net.preferIPv4Stack=true
Found java version 16.0.2
Exception in thread "main" 
java.awt.HeadlessException: 
No X11 DISPLAY variable was set,
but this program performed an operation which requires it.
        at java.desktop/java.awt.GraphicsEnvironment.checkHeadless(GraphicsEnvironment.java:165)
        at java.desktop/java.awt.Window.<init>(Window.java:549)
        at java.desktop/java.awt.Frame.<init>(Frame.java:426)
        at java.desktop/java.awt.Frame.<init>(Frame.java:391)
        at java.desktop/javax.swing.SwingUtilities$SharedOwnerFrame.<init>(SwingUtilities.java:1918)
        at java.desktop/javax.swing.SwingUtilities.getSharedOwnerFrame(SwingUtilities.java:1994)
        at java.desktop/javax.swing.JOptionPane.getRootFrame(JOptionPane.java:1689)
        at java.desktop/javax.swing.JOptionPane.showOptionDialog(JOptionPane.java:868)
        at java.desktop/javax.swing.JOptionPane.showMessageDialog(JOptionPane.java:670)
        at java.desktop/javax.swing.JOptionPane.showMessageDialog(JOptionPane.java:641)
        at net.minecraftforge.installer.SimpleInstaller.launchGui(SimpleInstaller.java:177)
        at net.minecraftforge.installer.SimpleInstaller.main(SimpleInstaller.java:147)
container@pterodactyl~ Server marked as offline...
[Pterodactyl Daemon]: ---------- Detected server process in a crashed state! ----------
[Pterodactyl Daemon]: Exit code: 1
[Pterodactyl Daemon]: Out of memory: false
[Pterodactyl Daemon]: Checking server disk space usage, this could take a few seconds...
[Pterodactyl Daemon]: Updating process configuration files...
[Pterodactyl Daemon]: Ensuring file permissions are set correctly, this could take a few seconds...
container@pterodactyl~ Server marked as starting...
[Pterodactyl Daemon]: Pulling Docker container image, this could take a few minutes to complete...
[Pterodactyl Daemon]: Finished pulling Docker container image
openjdk version "16.0.2" 2021-07-20
OpenJDK Runtime Environment (build 16.0.2+7-67)
OpenJDK 64-Bit Server VM (build 16.0.2+7-67, mixed mode, sharing)
:/home/container$ java -Dlog4j2.formatMsgNoLookups=true -Xms128M -Xmx2548M -Dterminal.jline=false -Dcom.mojang.eula.agree=true -Dterminal.ansi=true -jar server.jar
JVM info: Oracle Corporation - 16.0.2 - 16.0.2+7-67
java.net.preferIPv4Stack=true
Found java version 16.0.2
Exception in thread "main" 
java.awt.HeadlessException: 
No X11 DISPLAY variable was set,
but this program performed an operation which requires it.
        at java.desktop/java.awt.GraphicsEnvironment.checkHeadless(GraphicsEnvironment.java:165)
        at java.desktop/java.awt.Window.<init>(Window.java:549)
        at java.desktop/java.awt.Frame.<init>(Frame.java:426)
        at java.desktop/java.awt.Frame.<init>(Frame.java:391)
        at java.desktop/javax.swing.SwingUtilities$SharedOwnerFrame.<init>(SwingUtilities.java:1918)
        at java.desktop/javax.swing.SwingUtilities.getSharedOwnerFrame(SwingUtilities.java:1994)
        at java.desktop/javax.swing.JOptionPane.getRootFrame(JOptionPane.java:1689)
        at java.desktop/javax.swing.JOptionPane.showOptionDialog(JOptionPane.java:868)
        at java.desktop/javax.swing.JOptionPane.showMessageDialog(JOptionPane.java:670)
        at java.desktop/javax.swing.JOptionPane.showMessageDialog(JOptionPane.java:641)
        at net.minecraftforge.installer.SimpleInstaller.launchGui(SimpleInstaller.java:177)
        at net.minecraftforge.installer.SimpleInstaller.main(SimpleInstaller.java:147)
container@pterodactyl~ Server marked as offline...
[Pterodactyl Daemon]: ---------- Detected server process in a crashed state! ----------
[Pterodactyl Daemon]: Exit code: 1
[Pterodactyl Daemon]: Out of memory: false
[Pterodactyl Daemon]: Aborting automatic restart, last crash occurred less than 60 seconds ago.
 

1.12.2 supports Java 8, not Java 16.

Otherwise, the version you are using is no longer supported on this forum.

Please update to a modern version of Minecraft to receive support.

Currently supported versions are 1.19.4 (Latest) and 1.18.2 (LTS).