systemd cannot open display

I'm working on a program that that controls a trendnet Tk-409k KVM switch. The program is written in C++ with Gdk. The program runs fine if run from the terminal. I tried to make it a systemd service. It fails with a cannor open display.
After searching several sites. They say that printf and cout will write to the /var/log/messages file. I don't understand why I'm getting this error.

Thanks