In that photo, it appears to be missing/broken switch blade for an end of stroke switch on that score reel. And based on other recent posts here, likely is not vital to the operation of the reel or the game. Probably still something to fix eventually, but shouldn’t be critical to game operation.

Have you removed the reel from its rack and looked at the other switches in it? Those (or the reels being gummed up and needing a good cleaning) are more likely culprits I think.