Looks like you have not read the manual on both camera and lens.
If you give more details like the setup of your kit, are you taking a shot with the subject too near, light condition, etc.
i think people will then be able to give some tips.
the camera unable to lock the focusing, isn't it clear enough?
cause can be too dark, too near, subject keep moving, subject have no contrast or texture, shooting thur glass,highly reflective etc.....
yes, you can override it and able to take shots, but the shots mighty not focusing properly, is this what you looking for?
but.... but... if you half-press the 'trigger', how would the camera be able to capture a shot?
The shutter only activates upon a full press of the 'trigger' (shutter release) button :angel:
anyway, catchlights has already answered your question.
The camera can't achieve focus lock in autofocus mode, so it won't fire.
If you switch to manual focus mode, it will fire regardless of focus. But then you run the risk of focusing being out by a bit.