2001 Cadillac SLS Anti theft system

It has something to do with they theft deterrent, could be as simple as a faulty key so try a second one. It could also be the immobilizer which is on the ignition cylinder. The fact that is says service theft, would indicate there is a code stored. It would more than likely require diagnosis. Does the vehicle have any aftermarket stuff added to it like another alarm, stereo, or anything.