did you, by any chance, change lenses while the camera was switched on?
this seems a common way of getting an ‘error 99’ code up?
Never do that always switch off first. Like Damien said quite likely to be a problem with the aperture, mine worked ok most of the time f4.5 when it was at the widest end of the zoom and likewise when zoomed out fully it worked fine at f5.6. In between that it wouldn’t work.