That is the autofocus drive mechanism and it is supposed to be below the surface anytime the camera is not trying to focus a lens.
The lock/unlock pin is also raised as it is supposed to be; but you mentioned that the release button sticks in the down position. It is possible to bend the release pin so that it binds in the opening (I've done it).
Are you aligning the indicator marks (white dots)? The lens will only mount from one orientation.
Is the lens compatible? Some older lenses will bind on the aperture follower on the body (pre-AI lenses).
It appears that the mounting ring securing screws are all tight, but it is possible that there could be one (or more) loose/raised on the lens ring; and that could prevent mounting the lens.
It is also possible to bend a lens mounting ring (I've done it)... that can make mounting/unmounting very difficult.
Whatever the issue is; I HIGHLY recommend that you do not force a lens to mount, or mount a lens with a defective lock/unlock pin/button. If you manage to get a lens stuck onto the camera body the fix will be much more difficult and expensive (yeah, I've done that too).