[amd6547]

The beretta 21a has an ejector held on by a split pin through the frame.
It is not uncommon for the pin to work part ways out, and prevent the slide from coming off.
If this happens, you need to take off the grips, shine a bight flashlight up at the bottom sides of the slide, and try to see the pin sticking out. If you see it, try pushing the pin back in with a thin tool like a penknife blade of tiny screwdriver.
Look at a parts schematic to locate where the pin is at.