Why can't the Canon 450D shutter be pressed, and the focus is not accurate, so I can't take pictures?

The main reason why the shutter can't be pressed is that the distance of the still life you are shooting is closer than the closest focusing distance of the lens, which makes the camera unable to complete the focusing process, because the focused object is not within the depth of field of the lens focal section, which is also a reason why the shutter can't be pressed. Another reason is that the shutter is scrapped and relatively small. The detection method can adjust the control lever on the lens from AF to MF and try to press the shutter again. If the shutter can be pressed, it means the shutter is not pressed.