Shutter button is locked.

The shutter button is at the bottm left (in the picture) of the lens.
When time exposuring, depress the shutter button, and at the same time turn it clockwise. The shutter button will be fixed.
When not shooting, it can be locked by turning it counterclockwise and the red dot on it face the side.
In addition, shutter won't be released when the film is not loaded.