What kind of object is selected?
Is it perhaps already a guide? Or something else?
That won't work with live type objects.
You would first have to outline the type object (convert it to paths).
See Type menu > Create Outlines.
Is it also possible this same limit might be the same for "live" polygons?
I don't think so, Larry.
At least I'm able to convert polygons (live shapes) to guides.
Astonishingly, Illustrator is courteous enough to make them amphibious, that is: they are actually guides, but at the same time still live shapes that can be modified through the Transform palette.