The surface of your material has to be within 0.5" above where the surface of the tray would be. You may just need to put something up to bring it a little higher.
Tutorial: How to cut objects too thick to have the honeycomb in place
The GlowForge has a maximum material thickness it can cut through, but there is a maximum thickness you can even put on the honeycomb (crumb tray) even if you are surface engraving as the laser has a limited focus range of 0.01" - 0.4" *** above the surface of the honeycomb at the time of this tutorial. But let’s say you wanted to engrave something thicker that fits quite nicely inside the GlowForge but not with the crumb tray…