What does mean the message?

the Ubuntu system write a message on the command line after running the example Geant4.
What does mean the message mean? (see screenshot)

Mmmm. Not seen this before. Looks like an X11 issue. (X11 is the basic windowing system Unix systems.) Make sure you have the latest version of X11 installed.

Are you trying to run Geant4 remotely, i.e., on another machine and view it on your local machine? This requires X11 to be running on both systems. And if you log in to the remote machine with ssh, use the -X, or better, -Y option:

ssh -Y yourID@remote.machine

Sorry not to be more helpful.

Thank you for your reply!