loose spoke that will not screw back in
The spoke and nipple are turning together? This wheel uses straight spokes. Don't think it's likely, but the notch could be stripped at the hub, meaning there wouldn't be anything you could do about it besides get a new hub.

I think more likely your spoke nipple is probably stripped. Try to get another one from somewhere that is the same size as the ones already in your wheel, and try to replace the bad one. You just need to take off your tire and rim tape, and pull the bad nipple all the way out. Then get another nipple (2.0mm diameter) and thread on. See if it works. Could also be bad threads on your spoke.