Here is the error I'm getting when I try to print an report. But in my Ubuntu server I have installed "wkhtmltopdf".

Wkhtmltopdf failed (error code: -6). Message: The switch --header-spacing, is not support using unpatched qt, and will be ignored.The switch --header-html, is not support using unpatched qt, and will be ignored.The switch --footer-html, is not support using unpatched qt, and will be ignored.QXcbConnection: Could not connect to display