$DISPLAY is not set or cannot connect to the X server

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Error:

$ startkde 
$DISPLAY is not set or cannot connect to the X server.

Solution:
Do not use startkde directly. Use startx and configure it to startkde.

Make sure that X Windows is properly installed, configured and running.
Try:

$ startx
bash: startx: command not found

The result shows that X Windows is not properly installed.
Check your system installation guide to install X Server.

In you home directory, create a file named .xinitrc with this simple line:

exec startkde