I know exactly what happened here -the screw is frozen in its insert, which has broken loose and is now spinning with the screw. I've seen it before. This one is gonna be a tough fix.
Stainless steel (or chrome on brass) screws, when tightened into a brass insert without a generous coating of lube, will nearly always result in the screw galvanically locking to the insert. Bad news, as you have found.
Ordinarily, I would suggest penetrating oil overnight, and the judicious application of pin-point heat using a soldering iron applied to the screw head, followed by an ice bath to bust the screw loose. This has some risk, as it can negatively affect the plastic side plate. In any case, this removal technique will not work in this instance.
I would try the following - take out all the other screws and try rotating the side plate 180-deg. Then CAREFULLY wiggle the side plate while applying upwards pull on it. The idea is to use the side plate to pull the screw and its insert out of the spacer bar. See if that works.
Be very careful, because the last thing you want is to crack a side plate that is next to impossible to replace.
Let us know if this works, While I work on dreaming up some alternate strategy
Or, you might consider just leaving it alone, and working around the problem, when servicing the reel. That's probably the wisest course of action. At least nothing will be further damaged.
Sometimes less is more.