I just troubleshot this error. This is because of the DHCP service not having option 66 and option 67 configured. See this image.
The inside highlighted value for option 66 should be the static IP address that you are setting your FOG server on.
Option 67 should be (most commonly) undionly.kpxe
You can also make sure that the DHCP service is started on your server, and that TFTP is enabled on your windows machine(s).
I recommend following this guide, as it helped me tons. https://wiki.fogproject.org/wiki/index.php?title=FOG_on_an_Isolated_Network