Can't open display :0.0

This error message usually occurs when you are trying to run a graphical application on a Linux or Unix system, but the X server is not running or cannot be accessed.

To fix this issue, try the following steps:

1. Make sure that X server is running. You can check by typing the command `ps aux | grep X`.

2. If X server is not running, start it by typing the command `startx`.

3. If you are accessing the server remotely, make sure that X11 forwarding is enabled in your SSH client.

4. Make sure that your DISPLAY environment variable is set correctly. You can check by typing the command `echo $DISPLAY`. It should be set to something like `:0.0`.

5. If all else fails, try restarting your system.

If none of these steps work, you may need to seek further assistance from your system administrator or a technical support professional.