What does the error "NO X11 DISPLAY" mean?

When you get this error:

   "No X11 DISPLAY variable was set, but this program performed an operation which requires it."

It means that isCOBOL is trying to access the AWT/SWING GUI Interface and your system doesn't have that.

If your program doesn't have a GUI interface, you can try to set this property in your run command line:

   iscrun -J-Djava.awt.headless=true ...

or

   java -Djava.awt.headless=true ...

If your cobol program includes some textual ACCEPT/DISPLAY statements, you'll need to use isCOBOL's CHARVA toolkit by adding a '-t' switch to your command line:

   iscrun -t ...

This automatically sets this configuration variable needed to activate CHARVA:

 iscobol.guifactory.class=com.iscobol.gui.client.charva.GuiFactoryImpl