When taking in aperture-priority AE mode, set the shutter speed dial to "A".
(The shutter speed dial is lock when it is set to "A".)
Pressing slightly the shutter release button causes metering to meter and indicate the shutter speed in the viewfinder.
When taking manually, first release the lock of the shutter speed dial by depressing the button at the middle-top of the dial, and at the same time turning it if it's at "A".

The shutter release button can be locked by moving the lever at the base of the film wind lever over the red-colored dot.