Your red, green and blue cables from you set top box are either incorrectly plugged in or your setting is on the yellow composite input and you selected the other rgb component input. Check your cables and which one you are viewing.
It may also mean your settop box is not sending a component signal.
Either way the Slingbox is not getting a video signal.
Maxcat is correct; it is very possible that one or all of the Component cables are not plugged in properly. There are a few different things you can do to test this; I've linked an article below that goes through a few of the troubleshooting steps that you can try.
I hope this helps!