Light Fixture screw won't hold glass cover
You're right: there's almost certainly a nut floating up there on top of the fixture. The only way you're going to get it is to pull the fixture down a bit:


Note the two Phillips-head screws near the center, each in the narrow end of a keyhole slot. Loosen each of the screws about 1/2". When you're loosening the second one, the fixture will get wobbly
Turn the fixture about 15° clockwise, so that the larger end of the keyhole slots drop off of the screws and you can lower the fixture
Without straining the wires, and being careful where you put your hands, find the AWOL nut that's sliding around the top of the fixture.
Use the nut to re-fasten the central post to the fixture. Tighten it well! (Bonus points for putting thread-lock on it.)
Put the fixture back up onto the Phillips-head screws, rotate it counter-clockwise to lock it in place, and then tighten the screws
Now you can mount the cover back onto the post.