When my Canon EOS 1200D is set on AF, my camera seems to be not working. (I press the capture button and nothing happens except a few very quiet clicking noises (focus adjusting sounds). When I switch the lens to MF, the capture button works fine. Obviously I have inadvertently adjusted something—but I don't know what.
If your 1200D is in full Auto or P mode, the chances are that your camera/lens combination simply cannot achieve autofocus lock. The most likely possibility is that you're inside the minimum focus distance--that is, you're too close to your subject. The 18-55 kit lens has a minimum focus distance of 25cm.
In Auto/P modes, with autofocus set, the camera will not release the shutter until autofocus lock is achieved, and since you're too close for the lens to ever focus, that never happens. Once you switch to MF, the camera doesn't care if you're focused or not, and lets you take an out of focus image.
If it's not a matter of subject distance, then there's probably another reason you simply can't achieve AF lock. Maybe the light levels are too low (especially with a lens that has a smaller maximum aperture (f-number higher than 4), maybe your autofocus mode is non-optimal for locking on, or maybe you're shooting a scene with no contrast (white wall).