It might be hard to tell at first glance, but that locking plate has a thin sleeve pressed-fit over its hub portion. Use a small screw driver or pocket knife to carefully pry off this sleeve, revealing a tapered pin that must be driven out to free up the locking plate.
The pin must be driven out in only one direction. Look very closely and you can see one end is slightly smaller than the other. Use an appropriately sized pin punch and tap it out hitting the small end to drive it out on the large end side of the shaft.