I have no idea what this error means I have run through the installation for wkhtmltopdf
wget http://download.gna.org/wkhtmltopdf/0.12/0.12.4/wkhtmltox-0.12.4_linux-generic-amd64.tar.xz -P /tmp/ cd /opt/
What I have discovered is that gna.org has shut down (it was announced three months ago).
The domain wasn't resolving last week, today it is resolving but refusing connections. Thus you will not be able to download the file using wget. Thus it's possible you've got an incomplete / failed install.
You could try downloading it from a different page such as this wkhtmltopdf.org page.