I too have the same issues, However, my bodies are all solid as indicated by the icons, and i still cannot even select the base model or tool using combine, Nor can i select anything using the stitch too , telling me this is far more broken or i am missing something rudimentary that fusion 360 does . Its infuriating that i get no feed back from the software as to why i cannot select my body and tool for the cutout operation.
I have a Baseplate Body, and a Cutout for it, Both were made from an individual Sketches and extruded into individual Bodies. I Then Select my Baseplate Boyd and Clock on Combine in the Mesh Tab, It prompts me with the pop up and properties for the Combine Command and asks me to fist pick the Baseplate Body which will be cut out by the Tool. However I cannot click on anything on screen, nor can I select the body in the list of bodies, nothing, I try to select the Tool first instead, same thing nothing is selectable.
Same goes if I try to use the seam tool, or any tool that requires i select a body and a tool.
I have attached my File and screenshots of my working pieces.
Please help, If I cannot use fusion 360 for these simple boolean operations, its basically a brick to me.