I want to make a sub-assembly a reference in a drawing, reference only box is grayed out

Hello,

I have two large sub-assemblies inside a main assembly. I want one of the two to not show up in the BOM and come into the drawing in phantom lines.  I assumed all i had to do is right click on it in the assembly navigator, choose properties, and check the 'componet is refrence-only' box,.  However, this box is grayed out and will not let me choose it.

Any solutions?

Thanks in advance.