i installed odoo8 on debain testing. my wkhtmltopdf version is 0.12.1. when i tried to print sale order, this error message had showed up.

"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 "

don't know how to fix this.