"Unable to slice" message

[Delvo]

I was ready to print my first few test objects ever when this happened, so there's no "last known good configuration" to return to because I've never 3D printed before.

The same thing has now happened with two different (virtual) objects. I made my shape in FreeCAD, exported it to STL, opened Cura, opened the file so it's shown on Cura's image of what the print area would look like with my printed object in it, clicked "Slice"... and got this message:

Please review settings and check if your models:
•Fit within the build volume
•Are assigned to an enabled extruder
•Are not all set as modifier meshes

Well, I know it fits because I designed it to and I know the numbers, and the image that Cura is showing me shows that it fits.

But I don't have any idea what "assigning a model to an extruder" could even mean other than making sure the slicer knows what kind of extruder I have. And it came with a list of known printers to choose from including mine (Ender 5 Pro), and I've already told it which one I have, and it's already showing me information on the default hardware such as the nozzle diameter. Suspecting that maybe it just doesn't trust me not to have replaced something with a new part that isn't the Ender 5 Pro default, I also connected it by USB so Cura would have the chance to detect it, and that didn't make a difference. So I guess I'm presuming this one isn't the problem.

That leaves the thing about "setting a model as a modifier mesh". I don't know what a "modifier mesh" is, but I do know that it isn't a choice that came up when I was exporting to STL from FreeCAD, and I haven't seen it as a choice in any of FreeCAD's menus, so I don't have a way to export as not a modifier mesh (or know why this "modifier mesh" thing even exists if it make things unprintable)...